Found a strange phenomenon, not sure how to reproduce it but I'll leave it here for the record:
I usually start up osu!Post, then
manually start osu afterwards. Been doing this since early beta and had no problem at all.
Since a few days ago, I noticed these "strage" extra db files appearing in my osu folder:
...and my Collections isn't saving at all. After fiddling around and brainstorming tonight (thanks to Nathanael), I realised that something else is accessing osu!.db and that forces osu produce a dump instead of writing out the normal osu!.db file. I closed osu!Post before starting osu and that fixed the problem. I can have osu!Post running again and manually start osu just like before.
The problem simply went away and I'm unable to reproduce it again. I'll stick to my usual routine and report back if the problem returns.