KDE does have focus stealing prevention, Have you messed with that at all? https://docs.kde.org/stable5/en/kwin/kcontrol/windowbehaviour/index.html#focus-focusstealing The only thing I can think of is maybe some notification steals the focus and doesnt give it back to the window.
Anubis just released the no-JS challenge in a update. Page loads for me with JS disabled. https://anubis.techaro.lol/blog/release/v1.20.0/