check item 4 here:
https://steamhost.cn/steamcommunity_com/app/221410/discussions/0/1636417404917541481/

Unfortunately, i haven't been able to diagnose the issue myself even after reading that session of the discussion. I enabled logging and tried filtering part of the debug logs using tail -10 so it doesn't become a wall of text;

xxxx 5312 0 0.0 247756 5632 ? Sl 14:17 0:00 steam-runtime-launcher-service --alongside-steam
xxxx 5340 5.5 2.6 5030024 428500 ? Sl 14:17 0:21 /home/xxxx/snap/steam/common/.local/share/Steam/ubuntu12_64/steamwebhelper --type=renderer --no-sandbox --log-file=/home/xxxx/snap/steam/common/.local/share/Steam/logs/cef_log.txt --field-trial-handle=16091641918794305279,7966407460482870785,131072 --enable-features=CastMediaRouteProvider --disable-features=SameSiteByDefaultCookies --disable-gpu-compositing --enable-blink-features=ResizeObserver,Worklet,AudioWorklet --disable-blink-features=Badging --lang=en-US --log-file=/home/xxxx/snap/steam/common/.local/share/Steam/logs/cef_log.txt --product-version=Valve Steam Client --buildid=1705108172 --steamid=0 --num-raster-threads=4 --enable-main-frame-before-activation --renderer-client-id=4 --shared-files=v8_snapshot_data:100
root 5423 0.0 0.0 0 0 ? I 14:17 0:00 [kworker/u64:0-events_power_efficient]
root 5520 0.0 0.0 0 0 ? I 14:18 0:00 [kworker/2:0-mm_percpu_wq]
root 5521 0.0 0.0 0 0 ? I 14:18 0:00 [kworker/7:1-events]
root 5525 0.0 0.0 0 0 ? I 14:18 0:00 [kworker/6:0-events]
root 5565 0.0 0.0 0 0 ? I 14:18 0:00 [kworker/1:2-events]
root 5623 0.0 0.0 0 0 ? I 14:18 0:00 [kworker/5:2-mm_percpu_wq]
xxxx 5652 5.8 0.9 4714980 157052 ? Sl 14:18 0:18 /home/xxxx/snap/steam/common/.local/share/Steam/ubuntu12_64/steamwebhelper --type=renderer --no-sandbox --log-file=/home/xxxx/snap/steam/common/.local/share/Steam/logs/cef_log.txt --field-trial-handle=16091641918794305279,7966407460482870785,131072 --enable-features=CastMediaRouteProvider --disable-features=SameSiteByDefaultCookies --disable-gpu-compositing --enable-blink-features=ResizeObserver,Worklet,AudioWorklet --disable-blink-features=Badging --lang=en-US --log-file=/home/xxxx/snap/steam/common/.local/share/Steam/logs/cef_log.txt --product-version=Valve Steam Client --buildid=1705108172 --steamid=0 --num-raster-threads=4 --enable-main-frame-before-activation --renderer-client-id=8 --shared-files=v8_snapshot_data:100

I have also attempted to run the same game with proton 8.05,7 and experimental, in a useless attempt. Setting the d3d launch options does nothing. I am, sadly, out of ideas here

try moving the proton prefix folder for that game away from its current location, then play it again

when proton is unable to find it, it will recreate the prefix from scratch, potentially fixing a broken prefix (unfortunately this can happen, and then trying other versions of proton just doesn't help)

it should also restore the savefiles from steam cloud sync, but if that doesn't work, you can go to the old prefix folder and move them to the new one (that's why you should move it instead of deleting)

the prefix folder will have a c:\ drive folder in it and the subfolders structure inside it will look like a mini windows OS... that should make it a bit easier to know where you should search for the savefiles

if that doesn't help, post system info and proton logs as instructed in items [6] and [7] on that link, so others can have a look and maybe spot a sign of what's wrong for you

Sorry for the belated response, but I attempted moving the prefix folders and it didn't work. I later tried deleting them, also to no avail. I was rather busy this week, so I basically ended up uninstalling steam as a whole and didn't try anything else until today. This morning I basically decided to try to reinstall steam using the steam-installer package from cannonical repo again (instead of just using the app center) and it just started to work, about as inexplicably as it had ceased!

I don't think this will help anyone else very much, since i don't know what caused the issue to begin with, and I don't know exactly why it seems to have gone away ... the only thing that comes to mind was that i launched steam from the command line this time after the first install, and it finally decided to recreate the prefix folders. Thanks for the attention, and hope this helps someone else

ah yes... ubuntu can do that... i don't know why, but i remember seeing previous posts dedcribing a similar issue... installing from the gui app store has a different result from using the terminal to install what should be the exact same app through the exact same method

since i don't use ubuntu myself, i never really got a chance to pinpoint what's different netween those two

my guess is apt vs. apt-get or installing vs. not installing "recommended" (apt lingo for packages that are sort of like dependancies but not strictly necessary), or the snaps version may be preferred by the GUI without enough visual hinting (like PopOS used to do with the flatpak), idk...