Using "Time selection: Remove contents of time selection (moving later items)" with a non-quarter note length time selection while using tempo markers seems to completely break the action.
First demonstrating the correct operation with deleting a 1/4, then the incorrect result by deleting a 1/8.
Not sure what's happening here with the 1/8, but it makes no sense to me. I think either REAPER should add the correct 7/8 time signature there, or just ignore any tempo marker changes completely.
REAPER v5.33