I found a strange issue, which might be related to this:
https://forum.cockos.com/showpost.ph...&postcount=185
When I import a media item, duplicate it several times and press Undo (Ctrl+Z) all items disappear or get "minimized" respectively. Redo does not bring them back.
I'm on v5.978 at the moment, but the bug (?) also occurs in 5.91pre4.
I tried it with a default Reaper.ini file, but after some minutes the issue appears also.
It doesn't seem to be a general problem as it only affects some media items: