Fade out is not working on AE

You appear to be facing a problem that many people face with After Effects. Even though the keyframing technique you attempted is viable, the software is designed in such a way that it makes additional editing of audio more difficult than usual; hence, sometimes the fade effect may not act as anticipated. There is an option for using an audio fade workaround like the “Audio Fade” effect or some plugin for this issue but again it’s really not that simple as in Premiere Pro where you just have to drag the wave around for a second and then everything’s fine, right?
First off, I find your decision to switch back to Premiere quite curious. It goes to show how these tools created by Adobe can appear too complex at times even though they are very effective especially when used alone – switching from one tool of this kind to another will not be much help in overcoming such problems either (you cannot cross over from frying pan into fire). However, there are applications such as CapCut that are user-friendly in doing such tasks; they provide simple and quick fading out options which do not require one to manipulate many related factors individually. It is really unfortunate that Adobe has failed to make this process easier in After Effects while CapCut among others have been able to include them perfectly well. Let us hope that Adobe follows suit on these lighter applications and improves its editing of audio so that it becomes less clumsy in upcoming versions!

Ae has a particular way of fading/leveling audio. True. This is how I always do it:
Set a keframe to 0 dB where I want the fade to start, and then add a keyfram at -48 dB at the end. (Add Easy Ease to the first (0 dB) keyframe if you like that.) Ae's audio leves go down to -192 dB and that number is way to low. -48 dB is low enough to not hear it.
Here is an example of how it would look:

Depending on the frame rate of the comp a 10 frame audio fade may be impossible to detect.

In general in AE you would just animate the layer's audio level.

I did this just yesterday and have not encountered any bugs with this feature.

There is a bunch of bugs with this and this could be just such a situation. Since you know your way around Premiere it's the better option, anyway. In any case, do the usual: Reboot your system, clean out any disk caches. Perhaps it will work - for a while at least.

Mylenium