Those aren't mangled C++/CLI names, that looks like an obfuscator. It's not obvious whether the names directly are causing the issue, but I know Mono has trouble with obfuscated code.
This game has some C++/CLI modules so it barfs pretty hard on some mangled type names. [steam-263880.log](https://github.com/ValveSoftware/Proton/files/3096135/steam-263880.log)