Hi @Mustang519843
It’s a known issue which has been fixed for those on the SU15 beta & should be fixed for those on the current release build when SU15 is released, scheduled for the end of this month.
[Edit 05/04]
The release of SU15 was pushed back into May
[End of Edit]
Are you on the SU15 beta build?
Do you have the same issue if you follow the OP’s steps to reproduce it?
Yes
Provide extra information to complete the original description of the issue:
Are you using DX11 or DX12?
DX11
Are you using DLSS?
No
If relevant, provide additional screenshots/video:
Hi @HeavyWeather832,
Are you sure you’re enrolled on the Sim Update 15 beta? This specific bug has been fixed in the SU15 beta, and we’ve not seen any reports of it in the open beta since the implemented fix.
Thanks
The MSFS Team
Apparently I am not, but other than the screen issue the application appears to have improved.
I am on 1.36.2.0.
it’s particularily annoying with my weather window, i have to resize it every single time.
Same here. Makes me go bonkers.
Do you have the same issue if you follow the OP’s steps to reproduce it?
Yes, exactly, except I’ve never tried any beta versions nor am on one now.
Provide extra information to complete the original description of the issue:
Happens every time i restart the app, I’m using a second monitor on gigabyte RX 6750 XT with 7900X processor on windows 10
Are you using DX11 or DX12?
DX11
Are you using DLSS?
No
If relevant, provide additional screenshots/video:
Do you have the same issue if you follow the OP’s steps to reproduce it?
Yes
Provide extra information to complete the original description of the issue:
I always set the default to Full Screen, but starts 100% of the time in Windowed. I’ve found the .cfg and have changed it there but again it changes upon startup.
Are you using DX11 or DX12?
DX11
Are you using DLSS?
No
If relevant, provide additional screenshots/video:
Hi @AvatarAryk & welcome to the forums!
Although the custom window size is fixed in the SU15 beta (currently v1.37.17.0), it is not fixed in the current release build you are on (v1.36.2.0). So, you will continue to see this until the SU15 beta ends, and the current release build is updated to the SU15 final build.
FYI, the Sim Update 15 Beta forums are for discussions and reporting on the beta. It takes a while to figure out where to post topics - have a look at this topic:
And click here: Official Microsoft Flight Simulator > Forum Navigation for other great tips!
Hope this helps! 
@StarBlue1967
You don’t say what build you are on, but I’m guessing that you have not enrolled in the SU15 beta either?
Someone said that they have already fixed it for SU 15 
We must be very close to the public release of SU15 … if there are any beta testers following this thread, can they confirm if this issue has been fixed please?
@slicker55: Yes, it has been fixed for some time now in SU15.
It’s still happening for me - whenever there is a beta update it resets to windowed mode instead of full screen
This isn’t the same issue. The issue raised in this topic has already been fixed. When you get a beta update you may go into windowed mode, but you can just select ALT +ENTER to resume fullscreen mode. On all subsequent starts of the sim you will be in fullscreen.
Thanks
The MSFS Team
Just want to say I am having this issue where I can’t get Full Screen mode to stick after the offical SU15 launch. Both Alt-Enter and applying fullscreen mode via the sim result in a black screen and then obviously after seconds it reverts back to windowed mode.