I am putting this issue here in case some of you guys had same.
If I schedule a meeting in Thunderbird calendar and share/send to attendees, (1) the invited don't receive it and (2) when I open the event they don't show in the attendees list. If I use the browser to get into the underlying calendar of the gmail account and check, they do appear there though.
Everything was OK and working as usual until the last update.
In the event scheduling window the "Notify Attendee" check at the bottom is off and the text in grey - can't change it.
The latest released version of TB is 78.5 - are you using some beta version?
IAC, the calendar works fine with local 'home' calendars in later versions of TB. After 78.2, the Provider for Google Calendar extension, which provides read/write access to Google Calendars, no longer works.
Please confirm your TB version number, because 83.0 doesn't sound quite right. Is it just the version in the repos? Or did you install a beta version? [and what version of UM]