Problem appears to be due to WSE2 and its shader shenanigans which are not compatible with Proton.

I've done a full playthrough of TLD with WSE2 and there were no other apparent issues for that playthrough, thankfully. This is not a big deal, I know, but some suggestion for a fix or mitigation would be appreciated.

For any issues related to WSE, please report to the WSE dev directly. You can do so on the Taleworlds forum or on discord.

I did so awhile ago when I tried to figure out why HDR wasn't available and he suggested a page containing some launch commands to try but alas, none worked. It's no big deal though, TLD's stability has come a very long way and I no longer intend on playing TLD without WSE2's added stability as it's not really needed in this case.

Yes, TLD is designed to work well without WSE and thus we don't deliberately test it on that engine. So anyone using WSE should know what they're doing and beware that it's in their own risk.

Cool stuff that TLD can be played on Steam deck though. I wasn't aware.

I have to use a esoteric 'nox360' .dll to disable the built-in controller support and manually map everything to a template I've tuned to my preference, but I can play the game just as well as I did on my desktop. It's super fun!

Just updating this on the chance another Steam Deck user encounters rendering/compilation problems with WSE2 and is searching for a solution; I've found one.

Install reShade (reshade-linux, github) and link it with Warband in Konsole as you would any other game.

https://steamcommunity.com/sharedfiles/filedetails/?id=3300448391

Irregardless of reShade shaders even being enabled the rendering issue I originally posted is solved, along with WSE2's previously broken shader effects like FXAA, sharpening, HDR etc.

https://steamcommunity.com/sharedfiles/filedetails/?id=3300448204

Whatever the cause may be, redirecting Warband to use reShade's compilers and libraries corrects WSE 2's various graphical issues on Steam Deck, and potentially other Linux platforms.

For those wondering, I'm using GE Proton 9-4. reShade works with up to GE-P. 9-7 for most games, but fails to initialize with GE-P. 9-10 (tested). Note that GE-Proton isn't strictly necessary, as reShade can also function with Valve's Proton versions 8 to the current 9.0-2 but I wouldn't recommend them as an update from Valve could break existing support at any time, which is not the case for Glorious Eggroll. There are various tools available to easily acquire and install GE-Proton, like ProtonUp-Qt in desktop mode, or Wine Cellar in game mode (requires Decky Loader).

Also, I don't know why I used to think I needed a .dll to disable the in game controller support to assign a more sensible control scheme - that's a Windows problem totally avoided by Steamworks' Steam Input API on Linux. duh