I have E and R mapped to Mark In and Mark Out. For some reason, if I use the Merge Clips command to merge a video clip with its associated separate audio clip, the Mark In shortcut (and only Mark In) stops working, but only in the Source window, and only for the merged clip - it works normally for all other clips in the Source window, and works normally for all clips in the Program window.
The fix is to close the project and reopen, after which the Mark In shortcut will be working normally again for the merged clip.
If I then create another merged clip, the same thing happens - Mark In (only) stops working - but it will continue work normally for the previously-merged clip.
This is a completely repeatable, it happens every time I merge a clip. It's actually a longstanding bug that I've experienced in previous versions as well, but never reported before now.
Premiere Pro 24.2.1 (Build 2), MacOS 13.6.3