SUMMARY
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If a certain amount of icons are in the Icons Only Task Manager so that they don't fit on the panel, instead of resizing, one icon will disappear from the Task Manager. Switching to the app has no affect. Opening another app will cause the icons to resize as expected, and the missing icon reappears. Closing one app also results in the missing icon reappearing.

STEPS TO REPRODUCE
1. Open enough apps so that one icon disappears from task manager
2. Open one more app

OBSERVED RESULT
One icon will disappear from the task manager

EXPECTED RESULT
Icons in task manager will resize accordingly to fit

SOFTWARE/OS VERSIONS
OS: Arch Linux
KDE Plasma Version: 5.27.4
KDE Frameworks Version: 5.104.0
Qt Version: 5.15.8

ADDITIONAL INFORMATION

I believe it's exacerbated by the fact my Icons Only Task Manager is centered on my panel, but the same thing can be done with it in any position on the panel, so long as enough icons are present.

I have recreated this issue on multiple computers, running different OSes, but the same Plasma/QT/FW versions.

It affects both X11 and Wayland the same.

See my KReddit post for screenshots: https://www.reddit.com/r/kde/comments/17flfuo/weird_issue_icons_disappear_in_icons_only_task/