There is a problem with grouped tasks thumbnails in the icons only task manager and possibly the text variant as well. When "Clicking grouped tasks" mode is set to "Shows window tooltip thumbnails", clicking on a particular thumbnail can be tricky. I happen to find myself moving the mouse cursor through the shortest path to the thumbnail center, this probably is the case for many other people too. However, in the middle of the path often happens to be another task icon which makes the task manager change the tooltip window content and position, thus losing my target. One can workaround this by making first an upward movement of the mouse cursor and then to the side to the desired window, but this isn't ideal.
STEPS TO REPRODUCE
0. Use the task manager settings from the summary.
1. Have tasks A, B, C and D in the task manager. Tasks B and C are grouped together.
2. Click on the grouped task icon and then look for either the B or C task thumbnail and click on it.
If moved a certain way, you will lose the window thumbnail.
You shouldn't lose the grouped windows thumbnails by briefly hovering the mouse cursor over an unrelated task icon entry.
Linux/KDE Plasma: Debian unstable
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2
*** This bug has been marked as a duplicate of bug 358930 ***