Double post but:
To prove both our points further, I just had to do a custom Lutris install and configuration to get the old Bungie game 'Oni' actually working.
tl:dr - Modern, current (9/10) Proton can't handle .NET 2.0 properly, apparently, when I have a 64 bit system and its only made for 32 bit... and/or the engine that Bungie used for this is apparently ... essentially custom... theres nothing quite like it, according to the Oni2 people/website of people who've been reverse engineering it for like 20 years now and still haven't totally figured it out.
I had to jump down to the wine 8 custom lutris version, basically.
extremely odd.