Really? I could have sworn that when I move my camera around in my 5.1 setup the audio sources move along with the camera movement in my PMDG 737.
You probably need to be aware that engine noise isn’t necessarily coming from “behind you” this is because the sound vibrates through the fuselage and into the cockpit. So the engine noise could have been reflected and reverberate to around you and sound different compared to when you have a clear air between an audio source and your ears.
A good way to test is to switch to external cam and make sure your engine is running. Then use a drone camera to move your camera around, placing the engine noise source to the sides and behind you. If your rear speakers are not working at all, there’s probably an issue with your Windows sound mode. I know for a fact that my Windows speakers are constantly switching back to Stereo mode on its own without my knowledge and when that happens, my rear speakers just stopped working entirely. So I had to manually switch 5.1 on again and it works again, including in MSFS.
But if you move the camera around and the audio source is placed correctly in your speaker setup, chances are you should be good to go. And whatever sound and their placement that you hear in the cockpit, is probably the correct one already.