This is in Plasma 5.19.4 (Neon).

I haven't used the Breeze theme in months. I decided to switch back to it today, but if I choose the Breeze window decoration it seems to just switch to another theme.

At first it used one I had installed in userspace (Flat-Remix), although System Settings claims that Breeze is selected. I removed Flat-Remix to see what would happen, but now when I select Breeze I get Plastik instead.

I tried to reinstall the kwin-style-breeze package, but that didn't fix it…

What else can I try? Can't figure out what's causing this.

This is still happening. I couldn't find a bug report, so either I didn't search well enough or it's something about my setup… Any ideas?

It became worse. I tried to switch to the Breeze icon theme as well, and most of my icons disappeared! That continued even if I tried to switch to another theme again.

After creating a new user, logging in and seeing that neither problem existed for that one, I deleted all Plasma configuration from my home directory…

I had to reconfigure the entire desktop, but at least now themes work again.

Yes cuz kde neon is trash, i have spent 2 days to fix this **** but no results, no ways no feedbacks, no suggestions, only way is to delete it and install something else like kali, ubuntu or even windows