UT2K4 on Linux - Strange mouse glitch I can't figure out. Left click sends mouse pointer to top left?

Set up UT2K4 on Linux today. Game runs great, except when my mouse is captured by the game, every time I left click, the mouse cursor jumps to the top left of the screen. So I can get through menus just find by turning off the "capture mouse" setting and playing in windowed mode - but once I get into a game, I shoot my gun once and I'm looking at the sky!

I have tried EVERYTHING I can think of to fix this and I cannot possibly figure out what is causing it. I am unsure if this is an issue with Wine/Proton, an issue with my config, a hardware problem, or what! Never seen anything like it in my life. Hopefully somebody has heard of this!