Just noticed that, the "fullscreen" command in the window menu seems to be always greyed out in Qt/KDE apps. Even for apps with a working "fullscreen" command in the application menu (e.g. Konsole, Okular). The "toggle fullscreen" shortcut doesn't work, either.

Meanwhile, other apps (gtk, etc.) work fine.

Wondering, is this intended behavior?

I can only talk about my specific setting, but here Fullscreen is not grayed out at all on KDE apps, both e.g. Konsole and Okular have an active "Fullscreen" menu entry.

For the system details see the signature file. Unfortunately you did not specify your exact Plasma version or distribution...

My version is: archlinux, plasma 5.23.3, frameworks 5.88, qt 5.15.2. Seems only framework version is different.
Here my Konsole window menu is disabled:

While app menu works:

Nope, I apparently only forgot to update my signature since the last update, same Frameworks version.

Check if the same happens with a new user, if yes report this to your distribution. If no, something is wrong in your KWin settings

Thanks for the help!
I created a new user. Didn't have this problem.
Then I fiddled with various settings. Found that the problem occurs when the global scaling is set to 250% zoom in screen settings. Setting it 100% or 200% is OK. Seems a non-integer ratio zoom is bad.

(update: I tested in KDE Neon docker image + Xephyr. Same problem. And it depends on the screen resolution. 3200x2000 (my laptop) and 2000x1000 (Xephyr) have this problem, while 3000x1800 (Xephyr) doesn't.)