SUMMARY
Not sure which product to file this under, so please correct me if I filed it in the wrong place.
In the activities panel, apps from the taskbar can normally be dragged into the other activities. However, if multiple windows are grouped under the same icon/title on the taskbar, you cannot move just a single window to the other activity, you can only move all the app's windows at once.
For example, when the tooltip on hover is open that shows you the previews of the windows you have open, you cannot drag one single window from that and place it in the activity. That kind-of diminishes the point of being able to drag stuff into different activities, if you can't drag only the window you want and not everything else that's open in the same application.


STEPS TO REPRODUCE
1. Create two activities or more, and have a couple of (example: Firefox) windows open in the same app, and have window grouping enabled where all the windows show as a tooltip under the same icon.
2. Open the activities pane (Meta+Q by default)
3. Drag the app icon in the taskbar into the other activity open in the panel, and confirm that it works.
4. Drag just one single window from the hover-tooltip of the panel icon of the app into another activity. It does not work; nothing happens.

OBSERVED RESULT
A single window of an app cannot be dragged into another activity when the windows are grouped under the same icon

EXPECTED RESULT
You should be able to drag only one window into a different activity, as you could do when you don't have all the windows grouped under the same icon (example when you have the 'Task Manager' view enabled for the panel (right click near the end of the app list -> Show Alternatives -> 'Task Manager')

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.22
KDE Plasma Version: 5.22.5
KDE Frameworks Version: 5.85.0
Qt Version: 5.15.3
Kernel Version: 5.11.0-34-generic (64-bit)
Graphics Platform: X11