Looks like Steam is about to change its way to launch games on linux,
as recently (no idea when exactly, but can't be long) a new
command was added to the launch process (maybe only in steam-beta?)

Until now the commands "under the hood" were

run $HOME/.local/share/Steam/legacycompat/iscriptevaluator.exe legacycompat\evaluatorscript_435400.vdf
(launching the Steam-First-Time-Setup)

and

waitforexitandrun /$PATHTO/steamapps/common/HiddenFolks/Hidden Folks.exe
(command line options for the game launch)

recently the new command

$HOME/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=435400 -- $HOME/.local/share/Steam/compatibilitytools.d/SteamTinkerLaunch/stl waitforexitandrun /$PATHTO/steamapps/common/HiddenFolks/Hidden Folks.exe

was added between the previous command. There doesn't seem to be much info about the reaper process, but skipping it is no option, as it seems to be required (now) to launch the game.

(Hopefully the only) side effect is that the stl Wait Requester is opening twice currently, which needs to be fixed of course.
I also noticed that "manually" launching the Steam-First-Time-Setup started to segfault newer protons recently, which might be related(?).
Haven't checked yet if any env changed/was added.

If you have any info about this please feel free to share.
(I hope this is no sign, that possible tinker doors are about to be shut down before the big steam deck launch)