Windows 10 64-bit, Reaper 5.92pre2/x64
Noticed this one working today, and I thought I'd better make a report because I've never seen it before and would probably lose track of it.
I changed the default clef of a track and the notes were shifted in time to the right by what appears to be an exact dotted 32nd note.
Additional details: the midi item for the bass clarinet begins where the edit cursor is positioned, as you can see at the beginning of the gif. When the clef is changed, the start of the midi item is moved earlier. Time signature in this section is 9/8, if that makes a difference.
Also, performing an undo operation once will actually move the start of the midi item and the notes back to the correct position, but the clef change will not be undone until a second undo is performed.
I consider it fairly serious because it is possible to change clef of a track without any notes in view, and not notice that notes were moved incorrectly.