Created attachment 155520 [details]
Video capture demonstrating the issue.

I'm sorry if the title is convoluted, but I have no better idea on how to describe the issue succintly. Attached are video capture demonstrating the issue. 

SUMMARY
When set to only show icons from current activity, Task Manager will freaks out and show pinned apps from other activity which are not pinned in the current activity when pinned apps in the current activity are dragged to be re-arranged. Usually happens when apps "dislodged" by re-arranging also pinned in the other activity. The only way to "reset" is to switch activity back and forth. 

STEPS TO REPRODUCE
1. Enable "Show only tasks: From current activity" on Task Manager settings.
2. Have at least two activity.
3. Pin a several apps in each activity, each activity sharing some pinned apps but overall has different set of pinned apps. 
4. Drag to re-arrange a pinned app

OBSERVED RESULT
Pinned apps from other activity will "leak" to the current one

EXPECTED RESULT
Pinned apps should just normally re-arrange and no app icons from other activities that is not also pinned in the current activity should appear

SOFTWARE/OS VERSIONS
Linux: EndeavourOS
KDE Plasma Version: 5.26.5 
KDE Frameworks Version: 5.102.0
Qt Version: 5.15.8