Sorry for the time it's taken to come back on this.
So on one of the Catalina Macs, UTC was uninstalled and re-installed as part of a complete rebuild for a new user. This mac is running version 7.0.5 of UTC and experiences the font corruption.
On my own Mac which is running Big Sur, I uninstalled UTC, then opened an artwork in AI. I obviously had no font management for that artwork but also I didn't experience the font corruption in Firefox.
I have then reinstalled UTC up to the latest version of 7.0.10 and again opened some artwork files, and didn't have any font corruption.