A lot of dev companies treat their native Linux versions like an afterthought, if they don't outright outsource the project to a 3rd party. That's why the Proton version typically works better if the title isn't Indie. If an Indie dev makes the port, it's probably alright.

Admittedly, I think this is one thing about Linux gaming that Proton will make worse, now that devs can just test the game in Proton and not even bother with a native Linux version. I don't foresee that changing unless Linux somehow becomes the overwhelming majority OS, and while I think it will take a good chunk of market share in the coming years, I don't foresee it becoming the overwhelming majority for some time.