I have attached a video to show exactly what is happening because I don’t know exactly how to explain it. It started happening after the last major Manjaro update and it only happens to QT windows. GTK and other are fine.
Welcome to the forum! 
I could be wrong, but I seem to remember reading that it’s a Plasma bug, and that it will be fixed in a later Plasma update.
That said, I am not seeing this phenomenon here on my system, though. But then again, you are using a different window decoration, and this may have something to do with it.
What I would advise — albeit that I don’t know whether it will help in this case — is to log out of Plasma completely, so that you are looking at the login screen. Then, switch to a tty, log in as yourself, and delete the contents of your ~/.cache/ directory. And then, just so as to clear out anything remaining in memory, reboot. 
rm -rf ~/.cache/* && systemctl reboot
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Even before doing that - creating a new user to verify it isn’t a bad plan:
sudo useradd -m testuser
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Seems that the solution for now is to change the Window Decorations from Plastik to another. What folders inside the ~/.cache should I delete to reset the Plasma or Plastik W.D. settings?
Maybe Plastik W.D. is just broken in the current version of Plasma that I have (6.6.4-1)?
I do not know whether ~/.cache/ contains any data regarding window decorations per se — I haven’t checked — but it may contain other cached data referencing the chosen window decoration, and there is nothing in ~/.cache/ that cannot be recreated, so it’s always best to wipe it clean (while completely logged out of the GUI) whenever there are issues with Plasma.
Actually, it would appear that all window decorations except for Breeze are broken in one way or another — I do not have your particular issue, but I do have other decoration-specific issues — and I’ve already filed a bug report about it over at the KDE Bug Tracker after we switched to Plasma 6.6.4.
The irony of the matter is that Plastik is indeed also affected by the same issues as what I’m having, while it is still included in Plasma by default, notwithstanding that it’s a window decoration going back to KDE 3, and therefore bound to break compatibility with Plasma 6. And I’ve told them that too.
Going by my previous experiences with the KDE Bug Tracker, they’ll probably get in touch with me about this bug as soon as possible, i.e. no later than May 2030. 
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I’ve personally also experiences the brokenness of Plastik, I can’t remember what broke, but I changed window decoration and problem solved!
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