Yes, I have had wierd issues with Steam and external libraries. Mostly because of permissions and non-acsii characters in the path, but some have been unresolved. Steam is really complicated piece of software. The host OS, Steam linux runtimes and Proton all do their part. Flatpak can resolve and cause issues. That being said, external libraries are well tested on the Deck, and should work fine.

First step with troubleshooting would be logs. Check the Steam troubleshooting page on Arch wiki, and Proton Readme on Valve's github to get you started. You'll need to add PROTON_LOG=1 to launch options to get a log out of Proton. Also issue trackers for Proton and the linux Steam client are valuable resources.

I hope you get it solved.

Haven't run into this personally, but most of my gaming on Linux these days is on the Steam Deck without anything particularly interesting going on storage-wise.

It'd probably help with debugging if you add the distro you are using into the text of your post. Also, how are you launching the games? (Steam? Lutris? Heroic? Something else?)

For RE4 specifically, Steam has it listed as "Incorporates 3rd-party DRM: The Enigma Protector", so you might be running into some shittiness from DRM on that one, perhaps?

If you're running on Fedora or related distros, check your system logs to see if SELinux is complaining about anything. Sometimes the security features are overzealous.

Best of luck!

Are you logging your attempted game launches?

I wonder if the games are trying to search for files in a specific path, failing to find that path, and exiting. Or maybe something similar. The logs should tell you.