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You wouldn't have seen it in tests because you didn't test with firejail. I've mentioned that firejail is a sandbox program, which is a program that isolates programs for security purposes. https://en.wikipedia.org/wiki/Sandbox_(computer_security)

I tried turning it off, but it still does it, which seems strange.

>  If you can figure out what Unity wants from Firejail, I think that's your best bet in solving this issue.

I've been trying... which is why I've been asking you what dependencies you have.

This stackoverflow answer explains how you can find out about dependencies on .so files: https://answers.unity.com/questions/348751/what-are-the-stand-alone-linux-player-dependencies.html