Thank you all for your help and answers
It’s quite possible that the problem appeared on a previous version. I noticed those folders few weeks ago when the replication target’s Syncthing WebUI was restarting in loop - 2 or 3 weeks ago : I erased the index-v0.14.0.db folder, in order to let Syncthing reconstruct a working database. Then I saw those XXXXXX~1 folders being analyzed by Syncthing, I felt like those backup files were completely corrupted so I deleted everything, and let Syncthing do the full job again from scratch (to have a something clean on the replication target).
Then yesterday in the evening, I noticed that those XXXXXX~1 folders were back on the replication target, and coming from the source Syncthing (Windows side)
To answer some of the questions, yes, on the replication target, folders are appearing several times and are occupying several time the size.
What I’ll do this evening (100 GB of transfer are remaining, so it’s 97% completed, I’ll wait for it to be terminated) is to turn off Syncthing on the source side, erase the index-v0.14.0.db folder, and let Syncthing at version 14.52 rebuild a clean database.
If everything is fine, then, on the replication target, XXXXXX~1 folders should normally be deleted (transfered to .stversions) then I’ll carefully delete those folders by some “rm -rf” commands from the .stversions folder