I've tried REGEN, but not REGENALL and although the help file says REGEN and REGENALL do the same thing, I've noticed that this is not always the case. For example, when doing a RENDERCROP, REGEN will NOT remove the rendered preview whereas REGENALL does! I'll have to try to remember that the next time it happens.
FYI: I've noticed it happen in Paper Space; can't recall it ever happening in MS.