Thanks a lot for reporting! It may be because of the updated NW version of the engine, or maybe incorrect greenworks files? Also I don't have experience in Proton: do you perhaps know if games launched in that way are considered for Linux or Windows by Steam? I'll look for it.
Itch has the updated demo as well, does that work?
The new demo is just the itch version with some fixes. There isn't extra content, apart from a single extra banter over the spikes.