SUMMARY
In the icons-only task manager, if there's enough icons to make them be arranged into two or more rows, then the window previews that pop up when hovering the mouse over an icon will not be usable for windows that are not in the top row.

STEPS TO REPRODUCE
1. Use the icons-only task manager, and open many windows until the icons span more than one row.
2. In one of the applications that lies on a row other than the top one, open many windows so that it would trigger window previews when hovering the mouse over its icon.
3. Hover the mouse over the icon of the app from point 2, and try to switch windows using those previews.

OBSERVED RESULT
Impossible to switch windows, because as soon as the mouse cursor moves to the row of icons directly above (which it has to cross in order to reach the previews from a row other than the first one), the previews disappear.

EXPECTED RESULT
Should keep the window previews there so that it is possible to click them.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION