Look, you’ve been told what to do if you don’t want to see those artifacts, but I’ll repeat it one more time…:
- Change the window decoration;
- Disable the blur effect; or
- Live with it until upstream ─ i.e. KDE ─ fixes it.
Those are your three options. There is nothing the Manjaro developers can do about it. They only compile and package Plasma; they don’t write the code.
Given that this thread is going nowhere, I am closing it, and given that I’ve reiterated the available options again here in this post, I’ll be marking this very post as the solution.