Hi Andre - it's only happening when I sleep the displays. If there are any UI elements/interactions that might cause it, I've not encountered them. I did however, see the window shrink itself again while doing more troubleshooting.
I moved the display arrangement and the dock. It seems fairly consistent that the window will move (or move and shrink) in most screen configurations. Moved the dock to the right-hand display, swapped the monitor placement in Settings -> Arrangement and it also moved while on either screen.
The only time I can't seem to get the Usenapp window to move is when the arrangement is in my normal setup. Left display has the menubar (Making it primary), and the dock. If I move the Usenapp window there, it behaves itself. All other combinations seem to result in a move (or move/resize).
Again, let me know if there is anything that you'd like me to try, or logs to gather. Not a huge issue, but definitely something weird going on.