Yes, I fully understand. The problem is that I have some private data on my collection, that I'd prefer not to share if possible (I'll do if necessary).I tried to recreate the problem just by starting a new collection from scratch, and following the steps I usually do. No luck, everything worked as expected.
Then, after restoring my backupfiles on both devices, I'm, for some reason, unable to make the whole sync feature to work: PC simply gets stucked on "Sending database to connected device" forever. Don't know what can be wrong, I'll try to fix it.
Anyway, the problem seems to be, as you said previously, that for some reason, MS don't update the file info on the database after a full sync, that's why it keeps sending the files over and over.
An example with a problematic file. This is the info stored on the database (PC version):
And this is the same song (Android version) after a full sync PC > Android
This was one of files I changed on the PC using the "Swap file" command, but as you can see, for some reason the android database is simply not getting updated. Don't know if it's helpful, but it's what I've found so far.
PS: I'll keep trying to fix the sync feature again, replicate the problem with the full collection and send you the files if you need to.