Flatpaks fail to install with "Could not unmount revokefs-fuse filesystem"

[MichaByte]

Issue Description

Hi, I'm running the Orchid beta. I'm not quite sure what changed, but after installing a few flatpaks (LibreWolf, Thunderbird, etc.) I'm unable to install any more. I've tried KiCad, Discord, and standard Firefox. The error seems to occur after the app has been downloaded. The full error for Firefox is:
Could not unmount revokefs-fuse filesystem at /var/tmp/flatpak-cache-8V3EI2/org.mozilla.firefox-KCGJI2: Child process exited with code 1
I've rebooted to no avail. I'm happy to provide more details as needed.

Steps to Reproduce

• Go to GNOME Software
• Find a flatpak
• Click install
• (don't) Profit

On what version of Vanilla OS this happens?

Unreleased

Additional Information

Installed just hours ago, as of Jan 31 at 12:30 EST. This is a fresh download from early this morning.

[MichaByte]

This issue seems to be similar to #261.

[MichaByte]

flatpak/flatpak#4051 is related, I think.