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.