I think this is new in V5 because I've never seen it before. What does the word "synchronize" mean when it appears in a history step? I do not recall the exact phrase because I cannot reproduce the conditions under which it appeared. Having said that I do not want it to happen again because it caused me to have to delete and re-import a whole batch of photos (in order to start over).
I was editing some newly imported photos when I began to notice this "synchronize" message appearing in their history steps, and that edits were being applied to the wrong photos even though "auto sync" was NOT in effect. (only one photo was selected). In any event, auto sync does not cause this message (I just tested it). Does anyone know what function might have caused this?
(It is difficult to search the web for this issue because all the hits relate to auto-sync)