Proton 5.13-1 broken monitor handling

Affected games so far: Battlefield Bad Company 2, GTA 4, GTA SA, GTA V (testing the library in alphabetical order)

I have a dual monitor setup where i have a center screen and a left of center screen. In Proton 5.0-9 it would spawn the game on my center screen which is also set as primary, but in Proton 5.13-1 it now spawns on the left monitor, however the mouse region lock would be applied to the center screen.
On the example of GTA 4 and GTA V, it would spawn the launcher window on the center screen, but when the game starts, it fullscreens on the left screen while the mouse is locked to the center screen. Doing alt+tab twice would fix the mouse region to the left screen.
The other issue is that when alt+enter'ing to windowed mode, with the goal of manually fixing the monitor used, it will instead just minimise the game and when reopening it, it switches back to fullscreen immediately.

What do you require from me to debug this issue?

Start by reproducing the issue with a dual-monitor setup using the affected games, comparing Proton 5.0-9 with 5.13-1. Verify that fullscreen launches on the primary monitor, the mouse lock follows that monitor, and Alt+Enter can switch to windowed mode without minimizing or immediately returning to fullscreen.