After installing Cinnamon pattern alongside KDE, not only were Qt apps distorted (no icons, wrong theming, bad fonts) under Cinnamon, but also when I log back in Plasma session, the whole desktop was messed up. Changing settings through Plasma System settings did not produce any effect. I had to rollback to a previous snapshot. Not sure what other info can I provide for a valid bug report but I think it should be easily reproducible if anyone can look into it. I’ve included a screenshot of the Plasma session before the rollback with some KDE apps disfigured and icons missing https://ibb.co/x7005yV
https://ibb.co/x7005yV
I could be wrong, but this sounds like expected behavior. Trying to grill a pumpkin pie and trout in the same pan at the same time just isn’t going to work out well. Are you able to install Cinnamon and Plasma side by side in other distros, or is this solely a Tumbleweed problem?
To find out if the installation of both work out badly, or if it is only the switching between them of one and the same user, you could create a fresh user (or even two) and let the one use KDE and the other Cinnamon and then see if one or both have bad effects.