i havent changed anything about the synching itself, at least not intentionally. Does synching all the playlist tracks happen every time?
No it doesn't, it is very random. And for instance, I just retried a sync without changing anything neither on the PC nor on the phone since last sync, and this time, just one track got copied (it shouldn't have because it was already there...). The next time, nothing was synced (which was expected).
But you are right, I think I met this behaviour in the past as well, every now and then, hardly though.
And what do you mean by "to get them synced properly again" for your playlists?
The playlists themselves (the one that are copied at the end of the process, not the songs) could not be copied. I had to remove the lists from the device first (there were already there from a previous sync), then it worked and they got synced properly. And it works since... This was the first time I met this issue.
Sorry for not being much specific in the description, that's all I could find.
And by the way, what about renaming the device connected over MTP?