Hello @matthewdavis, it looks like this game has an empty Linux depot, so Steam treats it the same as a game that has a native Linux variant available. The empty Linux depot takes priority over the global Steam Play override, but not the per-game override.
You should be able to go to Steam -> Library -> right click on the game or the gear symbol on the right -> Properties... -> Compatibility -> check Force the use of a specific Steam Play compatibility tool to test the game with Proton.
@kisak-valve Thanks. That did it. Not sure if there is anything of action required to fix this, so I'll leave it open and you can close it if all is well.
proton experimentalx1 2021-07
Compatibility Report
System Information
I confirm:
Symptoms
Game will not install. Issue the install command. Succeeds. But game dir is zero bytes.
Reproduction
Just try to install the game using a Linux system.