This may not be a bug, per se, but it seems very odd to have seen the warning message (attached).
AFPUB 1.8.3; Mac OS X 10.15.3
I was editing and saving a document as I went. I had also saved sequential versions as I edited (to see if any problems arose). I keyed Command + S to "save" the file, did some more edits and keyed Command + S again and got this warning message. I should not have gotten that type of message as there is no other instance of AFPUB or any other process (including Design and Photo) that would access the document in question. That is to say there could be no reason (except bugs IMHO) that the message should have popped up. I did perform a "save as" as the message suggested.