Capcom have rolled out an update to MONSTER HUNTER RISE, and sadly it has broken it on Steam Deck. As they continue attempting to change their DRM in older games.
If the product you purchased no longer works on a promised platform due to a developer update you were sold a product that was not as advertised. Steam will refund you in this case, and it comes out of the developer's (publisher's) pocket.
Does that work for games run on proton. I don't think Capcom is making promises the games will run on Linux, so I wouldn't think this would count. The game does still run correctly.
Below two hours is the "no questions asked" interval. I believe what the above comment or was saying is that this is a valid refund beyond the 2 hour window.