Just to make sure why I brought this up, so not to possibly waste anybody's time:
It is not that I want to get these specific examples giving these specific results.
So it's not like "please help me getting this specific regex formula to work".
It's about trying to improve my regex skills, reading a lot, trying out a lot, and hoping to make use of some regex testers so I can experiment, validate, and fine-tune formulas in those, before copy/pasting them into MusicBee.
Running into this simple regex giving a different outcome in MB then it does in every tester I tried worries me about what will happen when using more complicated regexes.
I thought I either ran into some bug, or I was overseeing something very obvious. That's why I brought it up.