Office 365 Calendar has stopped syncing on Apple Calendar

Hi, 

I've gotten the issue that 1 specific persons calendar stopped syncing in the Apple Calendar app on Mac.

Lets call him Peter in this scenario.

Peters calendar works just fine on OWA and Locally in the Outlook App on both Windows and Mac.

However Peters calendar has stopped syncing events at 22/05/2019 in the native Mac calendar app, as i said this only happens to Peters calendar.

If i add Brians calendar it works just fine and it syncs everything.

If Brian adds Peters calendar its the same issue.

Calendars are being added under: Calendar -> Preferences -> Accounts -> "My office account" -> Delegation 

Both calendars are set to show all details within the organisation (Reviewer in PowerShell) so this shouldnt be the issue.

I've tried the following:

clearing the cache files of the calendar app.

rebooting many times

removing and re adding the calendar

tried multiple devices

tried multiple accounts to add the same calendar

tried changing calendar preferences in PowerShell and then change it back.

I am at a lost and would really appreciate all suggestions and input.