I have a question about GA planes. When I was flying in FSX and then XP11, as I gained altitude, I could feel the loss of engine power, the sound of their operation changed and I had to adjust the mixture flow with the mixture lever so that the engines worked properly at a given altitude and with the appropriate power. Unfortunately, this is not the case in FS2020, or at least I didn’t notice it, only when I push the mixture lever all the way down, the engines choke and turn off. There is no feeling of a drop in engine power as you gain altitude and the sound of the engines does not change. The rest of the engines’ timbre does not change when increasing or decreasing the gas. This happens on all standard and purchased aircraft. Do you have the same impression, or maybe there is some setting in the FS menu that changes it. I would like to mention that my settings are set to full realism and I tried to find settings that match this function, but I couldn’t find any.
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My first thought on this - check that Auto-Mixture is not enabled as an Assist.
no, auto-mixture is off
I´m just flying the DR400 in the Soldiers Bar area (85U to U60) and you definitelly see a loss of power, RPM decrease and sound change during climb. Indeed I had to perform ridge soaring several times to get some extra lift and be able to climb to FL100, as I was having serious problems to prevent stall while I tried to get out of the canyon.
Cheers
I just got into your DR400 and took off from Annecy towards Mont Blanc and unfortunately the plane does not respond to the mixture change, there are no visible changes in the engine speed or the timbre of its operation, only when you pull the mixture lever all the way down, the engine starts to choke. I don’t mean that the engine loses power with the altitude of the flight, because it does, but it does not respond to the mixture change lever. It’s almost imperceptible, and in FSX it was definitely noticeable
I know you stated your sim is set to full realism but just to make sure, could you check the automixture is OFF as commented by CasualClick ?:
yessssss 
what I mean is that regardless of whether the plane is at 1000 feet or 10,000 feet, the mixture lever is in the upper position or fully depressed like in a Cessna and then the engine has maximum power, and lowering the lever does not affect the engine’s operation unless you lower the lever all the way down, you will choke and turn off the engine. In FSX, as the flight altitude increased and the air thinned out, it was necessary to adjust the mixture lever to optimize engine operation. This is not the case in FS2020.
Ok, that’s weird because on my system the GA engine power is decreasing as expected maybe starting around 2500-3000 fts. Is your community folder empty ?
Also check if your flight model is set to “Modern” (not sure if it has an impact on engine power).
I have the same experience. In, for example, the default Cessna, I will definitely see my RPM begin to decline as I climb through 3000ft.
Reducing the mixture will restore RPM but must be adjusted as you continue to climb.
And this is not what happens to you?
as the altitude increases, the number of engine revolutions should decrease, the power should decrease due to the degree of air thinning, and by adjusting the mixture lever you supply more air to the engine (in increasingly rarefied air) you maintain the power at the appropriate level. In FSX it was very well simulated in FS2020, unfortunately not, regardless of the altitude at which you fly, the maximum engine power is with the lever set in the upper position
This is module specific behaviour. I see you posted under Skyhawk, so I assume your issue is with this module.
no, as I noted in previous posts, this applies to many GA modules, actually all of them. I no longer have any illusions, Microsoft screwed up this aspect of the simulation, nowhere is the influence of flight altitude on engine operation and mixture regulation well reproduced.
A couple of users have confirmed including myself that they don’t experience the issue you are reporting, so it should be an issue with your system setup, we are trying to help you. If it was an issue with the sim, it would have been reported by many more users and a bug report would exist for that.
I’m sorry, but all the users I discussed this issue with confirm one thing - there is no reaction to changing the mixture lever settings, apart from turning off the engine when the lever is fully deflected. I have no practical experience of how a GA plane behaves at higher altitudes because I only flew Wilga and at a maximum ceiling of 2,000 feet. However, I am an engineer and have been interested in aviation for a very long time. It is logical to me that as the flight altitude increases, the air becomes thinner and the need to feed the engine with the mixture becomes richer in air, which is why there is a mixture lever in the plane. In FSX, this mechanism was very well simulated, but unfortunately not in FS2020. If this problem has not been reported yet, I will be happy to report it, just tell me where. I am also sure that real GA pilots will confirm that this is how a real plane reacts.
When I climb in a 172 I definitely experience a decreasing RPM and loss of power until I use my mixture lever to thin the mixture. When at 10,000, my mixture is almost halfway out. I start seeing a decrease in RPM around 2,500 and begin adjusting my mixture. I am seeing this same behavior across all GA planes I fly in MSFS.
If so many users would have the same issue with piston airplanes, it is weird that no topic in the support or bug forum have been created. Since many users are real world pilots, they know what to expect about mixture control and so far there was no complaints about it, you are the 1st one in that forum (to my knowledge). I agree with the description you did on how it should work and we are not questioning that. On my system and as well as others that reported in that topic, mixture is working as expected, the more you go higher, the more RPM decrease and you need to adjust the mixture control gradually. So why on your system it is not working like that ?. If by any chance you could post a video (making sure your community folder is empty. Also try to run the sim in safe mode: the only way I know to start that way is once MSFS is running in normal mode, you need to kill MSFS.exe from your task manager so that when you restart the sim you’ll have the safe mode option.
The mixture system in the sim definitely works. I’ve got plenty of evidence of that. It’s a little overblown, actually too sensitive to power loss in some cases, but it definitely works.
As far as tambre, audible pitch only drops a few Hz for every loss of 100 RPM. This usually happens so slowly that you shouldn’t really notice in real time, but when you do discover you’ve lost 2-300 RPM and lean the mix back (regaining power in some cases), you will notice then. Offer not valid for constant-speed props.
OP likely has something else going on.
Well, oddly, he may be correct.
I say, “oddly” because I know that the mixture worked, if not correctly, then at least predictably before. I just ran a quick test.
C-172, Standard Day, Take off from KMCO.
Full throttle, mixture full rich, static run up. 2450RPM.
Climbout at about 100ft AGL, 70KIAS, 2550RPM.
Passing through ~3650ft MSL, still 2550RPM or so. Mixture is still Full Rich.
I climbed up to about 7000ft MSL before I had to go. Barely lost 50RPM and haven’t adjusted mixture at all.
Automixture off BTW.
All, my RL GA time has been tooling around down low, so I don’t know if this is accurate for a fuel injected C-172. I would say that it isn’t how this had been modeled previously, however.
Wasn’t there a recent change of the mixture physics in MSFS?
Could this be an improvement if the effect of mixture had been overexaggerated before?
Alternately, could this be what happens when you introduce new mixture physics but don’t retrofit your old stock aircraft to take advantage of it?
Don’t know. But at least for me, I’d agree with @BygoneBrake1515 that the mixture performance is different than it had been previously. 
Yes, in SU13 they have added new parameters for piston engine:
Continuing the discussion from Release Notes - Sim Update 13 [1.34.16.0] Available Now:
And I agree some planes maybe need some tweaks to use efficiently those new parameters.
Could you perform the same test with the stock C172 G1000 ?, I’ll also do that on my side. (I don’t have the C172 classic, just the C172 G1000, do they have different engine ?)
But saying that mixture is not working at all (for all GA planes) or just seeing a difference in RPM by pulling the mixture lever all the way down is incorrect, something is incorrectly setup there.