We'd already reached out to support without luck (engineering researching the issue), and this set of troubleshooting steps is one of the only results Google has on the issue.
Good news, your response inspired a little more digging and a final solution!
I'd advise anyone else with issues who has played with the Insider Fast Build feature and then disabled it, please re-enable the feature then apply the newer versions of Office -- at least getting you to version 16.24.
The version we were running is 16.23 as the client had been using the Insider Fast Build (not on my behest), and while troubleshooting I'd switched back to the Production/Insider Slow settings and the reinstall somehow put version 16.23 of Outlook back -- perhaps because I still had those versions of Word, Excel, Powerpoint loaded.
Bottom line is once our version matched one of the ones you mentioned above (16.22.1 or 16.24) the issue was resolved.
Thanks again,
- David