Try setting TMPDIR to /var/tmp

[oskarkook]

As described here: https://github.com/flathub/flathub/wiki/App-Requirements

This should affect the ability to open the files stored in an archive.

[oskarkook]

Note that here, it says:

If an application uses $TMPDIR to contain lock files you may want to add a wrapper script that sets it to $XDG_RUNTIME_DIR/app/$FLATPAK_ID.