Some Events Not Syncing With All Apps

I'm not sure about the "new security", but MacOS and Microsoft 365 are both right up to date. Today I removed calendar from the exchange account sync and re-added it. It took some time, but all of the calendar entries populated. I then added a new event via Outlook and it didn't update in Calendar app. I changed the refresh calendars setting for the account from "Push" to "every 1 minute" and it then copied the event across. I created another event in Outlook, and another in the Calendar app, and neither has been copied across. Updates from Outlook have appeared on the iPhone app, but not the event added in the calendar app. I've tried changing the refresh setting again, but it makes no difference. So the updates in both directions don't work.