Fullscreen video broken on wayland with multi-monitor

[guido-pe]

Ever since the latest round of updates on my computer, fullscreen video playback is broken for me when using either mpv or vlc. Fullscreen playback using dragon player still works, but dragon has pretty bad image scaling. Fullscreen video playback also still works using Firefox.

When I play back a video with mpv or vlc and then make it fullscreen by pressing "F" or double clicking into the video, the left monitor will show the left half of the video, scaled up as if trying to play it over the area defined by both monitors, while the right monitors shows the whole video, but scaled down from the original size (the video is originally smaller in both dimensions than any one of my monitors), with the rest of the monitor switched to black. If I the move the mouse cursor to the right monitor, they will switch behaviors, with the right monitor showing the right half of the video scaled up, and the left monitor showing the whole video, scaled down.

The display server used is kwin version 6.1.3, mpv is version 0.38.0-r1 and vlc is version 3.0.20-r11.
My GPU is an AMD RX 5700 XT. My monitors are both the same make and model, both set to 2560x1440, and both connected via DisplayPort.

My current workaround is to start the video in mpv, maximise the mpv window using window manager controls, then manually hide the window decoration.

Does anybody have an idea what might be going wrong here and how to go about fixing this?