This is extremely common. There's a vanishingly small number of games that officially support the Steam Deck that do NOT unofficially run on any given Linux box. That small number seems to be exclusively gacha games. A number of those can be made to run by setting `SteamDeck=1 %command%` as the launch command.

Anyways, BG3 runs perfectly fine, natively, on my Ubuntu 25.04 RTX 4090 rig.