Outlook Calendar Groups Malfunctioning

I'll save anyone reading this thread some time - these are the two solutions that have worked for people:

For me, it worked to repair my Outlook installation:

It seems people have misunderstood my original solution. I did not "re-install" Outlook, I "repaired my installation" of Outlook. Those are two different things. I haven't had the issue since then. See this link to learn how to repair the installation: https://www.slipstick.com/tutorial/repair-office-outlook-installation/#:~:text=The%20steps%20are%20basically%20the%20same%20for%20all,the%20desired%20repair%20option%20and%20complete%20the%20wizard

For others, it worked to uncheck "Turn on shared calendar improvements".

1. File
2. Account Settings
3. Account Settings (Again)
4. Email Tab
5. Double Click Your Email In The List
6. More Settings
7. Advanced
8. Uncheck "Turn on shared calendar improvements" Under "Microsoft 365 Features"
9. Click Apply
10. Click OK
11. Exit Outlook Entirely
12. Restart Outlook

If it doesn't work after these steps, restart your computer and then check again before assuming this step hasn't worked. If another solution is found, I will edit this comment with that solution.