Forum Discussion
Some Events Not Syncing With All Apps
Other than what you've done, you could be having a security issue and it's glitching. Do you have business 365 or an outlook.com/hotmail.com account? If security is not tight, the bumps happen.
Both of the synced 365 accounts are Business and both are configured with MFA. I do have an Outlook.com account (as a recovery email for my gmail account), but it is not synced with Calendar. I have no issues with email for the business accounts - just calendar sync. If there were security issues, I would expect to see some form of error somewhere, but it may just be that they never considered that users would need to see them. I think it would be interesting to try another calendar app to see if the same issues occur there (it would have to be using it's own sync engine though). It might help point to where the problem lies.
Ian_Lewis​ since it's a business account, you could put a ticket in with MS and request the Mac team. I'd be very specific about the issue with the Mac calendar only. This helps in their ticket system to route it to the right team. I would not mention the other accounts as it's not necessary. The key to calling MS in my past experiences is keep it simple in the request. If this is a true bug, they will figure it out. If it's something on the Mac, then they will send it to Apple to look at (meaning you call Apple). It could be something wonky on either side.