“Unfortunately, Flatpaks can come with extremely liberal default profiles that make the sandbox mostly pointless.”
Ralf dropped Firejail because its profiles run to many hundred lines of interlocking allowlists and blocklists and break on updates. His fix for Flatpak is a global override that strips home, host, SSH and GnuPG access from every app. The official advice for VSCodium with LaTeX is to switch the Flatpak sandbox off entirely. A sandbox that ships open by default is a label, not a sandbox.