Can’t tell specifically what is making this happen but I have to restart to make it work again. Just editing this time, no messing around with glyphs or masters. I did do some cut/copy and pasting though.
If it happens again, please try to remember what you did to that glyph. The undo is glyph based, so if it breaks for one, it will still work for the others. And the damage can be done long before you recognize it.