Certain games on Steam that you play across either Linux desktop or Steam Deck get a specific version of Proton set by Valve, and at times they don't quite work right like ELDEN RING.
In this case Valve had set Proton Experimental to be the default to run ELDEN RING, where it did work previously, however they just recently upgraded Proton Experimental basing it upon the newly released Proton 8. With any new Proton version can come breakage as the translation layer isn't quite perfected just yet and in this case the popular game is having issues with the Proton upgrades.
As mentioned on Twitter by Valve developer Pierre-Loup Griffais:
The launch issue with ELDEN RING has been addressed by temporarily reverting the game to using Proton 7 while the Proton 8 update issue is being investigated. Restarting Steam will pick up the fix.
So if you're forcing a compatibility option on ELDEN RING, you might want to uncheck it to pick up what Valve set or change it yourself back to Proton 7 to have it work again.
Nice to see Valve continue to keep on top of compatibility like this.