I don't know. I did some retries, checked if everything was harmonic among the several machines, like encryptions (not enabled), sync delay, Joplin version. Yes, everything was compatible one with the others. When I tried to sync one more time this morning, it did work. So, for the time being, I can't help.
By the way: I am using syncthing, so I checked that too. Seemed OK too, but I know that some times (not very often) syncthing will forget some files behind for a time and then pick them up later. So I can not discard the possibility that the issue has to do with syncthing.