Hey folks!
I've had this issue for ages and I think I finally figured out what causes it:
I usually create empty items, place Video FX on them for text, drawing boxes, and so on. Then I would use item envelopes to transition them in.
The issue, when the playback rate of an item is over 1.00, then whatever the Vid FX is displaying randomly disappears. Setting the Item playrate back to 1 will fix the issue, though it messes up automation.
I know REAPER is not supposed to be a video editor, but it is for me, so I'd love to see a fix for this
Alternatively, a feature request for razor edits to work on Item automation but that's a whole other bag