wait i just realized, you sent me an angry comment when my GPU was broken and I wasnt even using my computer. Now im really confused. What did I do to hurt you? lmao

what are you even on about? lmao i just saw your comment now

this guy is alright...

Ok. Solved. The music itself was the issue. As far as i tested, for whatever reason, i needed to re-encode the mp3 file with Audacity with an average quality of 192 kbps. It works now, but i need to do it with another 110 files. Welp. Thanks a lot for your help to pinpoint the problem, at least it wasn't a proton or Steam Deck issue.

Don't feel obligated to experiment or something like that, but i would definetly say "Thanks a lot for your help" if you do. I have tried what you just have posted (game save is cloud save, the custom bgm i'm using it's the same i used on windows and worked in ther, assigned custom track to a random unit and restarted the game) but still no success. Can't help but wonder what i'm doing wrong, or what you did right xD

I had imported my cloud save and Custom BGM folder from Windows, and when I reassigned Devolution Getter's BGM I had to relaunch the game to make it use the new song. I can experiment with it more, if you need me to.