The game is completely offline, so, from a Windows standpoint at least, playing the game in offline should not influence how achievements are unlocked. Perhaps this is an issue or behavior on Proton/Steam Deck-side?

This all sounded so weird to me, but you're right. Thank you! :)

I've received plenty of achievements playing other games offline with my Steam Deck, but somehow, not in this game. No idea why.

Just got a Kud cheevo playing offline on my desktop, so I guess it's not offline in general but a weird Steam Deck specific thing. My disassembling days are far in the past, and I'm too lazy to try and figure out what's going on anyway. :)

Feels like I should report it to Valve somehow, except for the fear they "fix" it by randomly banning another visual novel...