I get what you're saying, I used KDE even back in the 2.x days. I could argue that this is kind of different though; it's not only third party themes, it's everything based on Aurorae. For example, Plastik comes with Plasma by default. Guess what, also has an issue.
First thing I learned with KDE was to definitely fuck around with the themes, it's one of the great things about it. Was just looking around, seeing nobody else has reported something similar to this, hence I wasn't sure whether it's a 6.0 regression or whether it's something unique to my system. If I wanted a system that others are making the decisions from me, I'd use other DEs (of the garden ornament variety).
This was fixed for me since 5.27 but it resurfaced with 6.0. Breeze/Lightly/etc are fine; it's Aurorae themes with fractional scaling in Wayland that have the issue. Not sure whether it's a bug or something left over from somewhere in my system.
I am mainly using Strawberry - I keep away from Elisa because it can't handle internet playlists (like my di.fm favourites etc). I have fond memories of Amarok, so hopefully it'll be great!