SUMMARY
In JOSM (Java OpenStreetMap editor), there are several subwindows that are opened when performing actions like downloading or uploading map data. These windows work properly the first time they are opened, but every subsequent time, the window's contents are not updated when they are opened. Interacting with a widget in the subwindow, like scrolling in a listbox or panning a map, will cause that widget to be updated. Resizing the subwindow will cause all of it's contents to be updated.

This only occurs in Kwin (X11), even with the compositor disabled. I've tried Gnome & LXQt, but the issue doesn't occur for either. Haven't tried Kwin on Wayland.

STEPS TO REPRODUCE
1. open JOSM & add an imagery layer so you know where the camera is (Imagery > OSM Carto)
2. click File > Download Data - a subwindow with slippy map opens with the view of the main window selected
3. click Cancel button to close subwindow, move the camera in the main window, then reopen it 

OBSERVED RESULT
It is only updated after performing an action that causes the subwindow to be redrawn, such as panning, zooming, or selecting an area in the slippy map, or resizing the subwindow.

EXPECTED RESULT
The slippy map should be updated to show the new camera location when reopened.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 5.20.2
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1

ADDITIONAL INFORMATION
Possibly related to #380389