SUMMARY
Maybe i'm in the wrong box to post this, it took some time to figure it out, but finally caught it, hope this helps ;)
(This wasn't an issue with Plasma/X11, I experienced something like this with Plasma5+Wayland earlier, but couldn't yet reproduce back then)

On a multi-monitor setup, Plasma 6 task manager becomes unresponsive for user events while one screen is in fullscreen mode under Wayland.
Leaving fullscreen gets it back to normal.

STEPS TO REPRODUCE
1. Start an application with fullscreen capabilities (tested with: (s)mplayer or any browser with youtube)
2. Start another application on a different display (tested with: Konsole)
3. Make the 1st application switch to fullscreen mode
4. Switch to the 2nd application on the other display. After a few keystrokes or seconds:
task manager stops updating and even KDE shortcut keys (volume up/down) stop to work.
(The keystrokes in Konsole still do work)

weirdly
A: during the "freeze" being Konsole on focus, "switch keyboard layout" shorcut key applied properly
(but without task manager updating the layout status)
B: Mouse navigation/focusing between applications still work
(but task manager does not show task focus change)
C: Keyboard based application focus switch (Alt-Tab) triggers a one-shot update on task manager
(updates task focus and the clock gets updated once to the current time)

5. Switch the full screen application back to "normal mode" and instantly some of the events during this weird freeze "got played"  (volume up/down events happen, Plasma popups on keyboard layout/volume do appear)

OBSERVED RESULT
On multi-monitor setup, a background full screen application (running unfocused) somehow hangs the task manager updates/kwin shortcut keys on focused application.

EXPECTED RESULT
Fullscreen mode on one application should not stop global keyboard events or task manager from updating.

SOFTWARE/OS VERSIONS
Fedora Linux 40
KDE Plasma Version: 6.0.5
KDE Frameworks Version:  6.2.0
Qt Version:  6.7.1

ADDITIONAL INFORMATION
NVIDIA 1080 graphics card with proprietary drivers installed