Thick window borders why?

Using KDE on Tuxedo OS, most themes I try have abnormal thick window borders all around. Setting no borders in window decorations does not help.
Breeze and Tuxedo are normal. The themes don’t have thick borders in their screenshots.
What could cause this?

Do you have a screenshot?

Seems like a real issue; I’ve seen people on Reddit complaining about the same thing.

What version of Plasma are you using? Are you using X11 or Wayland? And can you provide an example of a theme that exhibits this issue?