This issue is not resolved. At least not on Plasma 6.

Some windows (e.g. undo history) will pop up at the origin of the underlying coordinate system of the displays – which is the top left corner – every single time, even after having moved them before. Neither the intended behavior nor the intended initial position, I presume.

If the primary monitor is to the right of the secondary, affected windows will thus open in the top left corner of the secondary screen, even when the unity editor is on the primary screen.

This circumstance has the side-effect of causing affected windows to initialize outside of visible screen space if the secondary monitors y-position is shifted downwards further than the height of the window.

Because I can't attach a visualization here I want to give an example:
Let's say the undo history window has a height of 500px and the secondary monitor has the coordinates 0,600. The window initializes at 0,0 – making it invisible and unusable.

This happens on X11 and due to Unity relying on XWayland on Wayland as well.
Also happens on a fresh user with pristine configs and settings.

Yes, the popup windows launch were they were last used.
The problem is when you have multiple monitors. The popups DO NOT move with the application.
If you move Unity from Monitor 1 to monitor 2, the popups (hamburger menu's), context menu's, Add Component menu, gizmo's menu, scene menu, game menu....
Still appear on monitor 1.

ONLY the top level (File, Edit, Assets, GameObject, Component..._) menu's work properly.

Finally, unlike build settings window, context menu's cannot be moved to a new location.