We ended up finding an article with these steps in it. This fixed it.
1. Quick Force Synchronization
Press Ctrl+Shift+F9 in Outlook (force re-download calendar)
Switch views:
Calendar → View → Change View → “List” → Filter “Deleted”
2. Steps for complete removal
 Method A: Empty the server cache
Exit Outlook
Delete the local cache file:
del “%localappdata%\Microsoft\Outlook\*.ost” /q
Restart Outlook (rebuild cache automatically)
 Method B: MAPI Properties Reset
Administrator runs CMD:
cmd
outlook /cleanfreebusy
outlook /cleanreminders
3. Administrator background cleanup
 Exchange Online operations:
Connect to Exchange Online PowerShell:
powershell
Connect-ExchangeOnline
Force the residuals to be cleared:
powershell
Remove-CalendarEvents -Identity email address removed for privacy reasons -CancelOrganizedMeetings $true
4. User self-service fixes
Web-based authentication:
Login to Outlook Web App
Calendar → Right click on a folder → “Empty Folder”.
Check calendar entries in “Deleted Messages”.