ALT Tab (change task) no longer works while dragging a selected string

[ñull]

SUMMARY
This seems like a functionality regression. Previously I could select a text, start dragging it, then with ALT Tab change to another applicaton window and there drop the string in that window. I don't see the point why I should always contaminate the clipboard with strings when I could drag them like works in other applications. Work around is slow and needs more coordination and finding the right one, namely to drag the string to the desired task tab in the task manager which will make it change the focus there. When that still works, why not ALT Tab like before?

STEPS TO REPRODUCE
1. Select text string in task Firefox (or Thunderbird )
2. Start dragging while changing to previous task using the ALT + Tab key combination.

OBSERVED RESULT

ALT Tab will no longer change to previous task during a drag operation.

EXPECTED RESULT
ALT Tab will also change task while doing a drag operation.
SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 20.04.1 LTS standard Plasma repository.
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION

This was first posted as Plasma bug (https://bugs.kde.org/show_bug.cgi?id=430371) but the community came to the conclusion that this bug is only seen in Firefox and Thunderbird.