SUMMARY

I ran into this using Okular, but I believe the bug is not Okular specific.

In a Plasma Wayland session, it is possible to drag a Qt toolbar outside the toolbar area, just like in X11. However, when the toolbar detaches and becomes floating, it is no longer possible to drag it or re-attach it to the docking area.

I found this workaround: change the toolbar orientation to left or right to make dock again.

Moreover, the detached state is saved and, upon restart, the floating toolbar is invisible (possibly a separate bug? There's a workaround for this too: toggle the toolbar visibility in the Settings menu and the toolbar will re-appear.

STEPS TO REPRODUCE
1. Open any document in Okular. I used a JPEG image.
2. Right-click on any toolbar to open the context menu
3. Uncheck "Lock Toolbar Positions"
4. Grab a toolbar and drag it outside the docking area
5. Release the toolbar and try grabbing it again 

OBSERVED RESULT
The toolbar can't be dragged any more!

EXPECTED RESULT
It should work just like in X11

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.91.0
Qt Version: 5.15.3-1.fc36

ADDITIONAL INFORMATION

This is Fedora 36 one week before the official release, but I don't think it matters.
Not sure why Fedora 36 is releasing with KF5 5.91 instead of 5.93... THIS should matter.