POP_OS: Stam VAC Anti-cheat prevents me from playing DOTA2

I migrated almost everything on Linux and, low and behold, the only game giving me issues is DOTA2.

I'm using Steam flatpack, it runs, I can play against bots, I can spectate games. The moment i try to play in matchmaking a VAC messadge pops up preventing me from starting a game.

I tried reinstalling, veryfying the files, installing on a differend hard drive, scanning the disks for errors.

Nothing worked. Any idea?

Edit: thanks guys, it was actually Proton!