When you time remap the last keyframe is placed after the last frame of the footage or the composition. If you have layers that extend past the last frame of the composition then you will not see black, but you will see those layers. This is normal.
I use time remapping all of the time and I have never run into any issues that were not caused by user error. If you want to make a loop you need to the last keyframe (k key), then move back one frame (Ctrl/Cmnd + left arrow), set a new keyframe, then delete the last keyframe. If the first and last frames are identical then a simple loopOut() expression will create a perfect loop. If the first and last frame of the sequence is not identical then you need 3 keyframes to create a looping pattern. The third keyframe is placed right after the last frame in the sequence an is a copy of the first keyframe.
The only time I have ever seen anything resembling what you are describing is when folks do not follow the workflow that I have explained.
Still having this issue? Or is it fixed? Let us know.
I'm still experiencing the issue sporadically.
The issue is not a misunderstanding of how to loop a composition with time-remap as Rick suggests.
The issue shows up sporadically and goes away without actually changing anything.
I'm going to re-install the app, as not sure what else to do at this point.