Your system information

Steam client version (build number or date): beta 2021-09-29
Distribution (e.g. Ubuntu): Gentoo Linux
Opted into Steam client beta?: [Yes/No] Yes
Have you checked for system updates?: [Yes/No] Yes

Please describe your issue in as much detail as possible:

Yesterday, after I rebooted my computer and launched Steam, I noticed a high system load and high IO activity. Watching the process monitor, it looked like Steam reinstalled each and every Proton version that existed in my compat tools folder (including the official ones) into a prefix with appid = 0. It spawned wineserver, bwrap, pressure-vessel and steam-runtime processes repeatedly for each Proton version in my Steam installation causing a high system load in the process. It also asked if I want to download and install the wine-mono 6.4 package, and complained about a "file not found" issue (both dialogs were native wine dialogs). This whole process took about half an hour, during this time, it was not possible to launch other games: They would just sit in starting state with Proton showing a dialog that it is starting the game. Even after the system calmed down again, Steam didn't launch the game queued for starting, I had to force-close it, then tried again and it worked.

Steps for reproducing this issue:

Start your computer
Launch Steam Beta from an installation before version 2021-09-29
Watch htop to see wine prefix installation for multiple Proton versions into appid = 0