NM: What is Calendar Repair Assistant (CRA), and why it should be disabled during migrations (4329118)

Calendar Repair Assistant (CRA) runs and reviews the chair and invitee's calendar, if there is no matching meeting between the two calendars, CRA will replace the meeting, thinking the deletion (missing meeting) was done in error and requires repair.

Calendar items should never be deleted from the target mailbox. CRA thinks a Delete is a mistake on the user’s part, and will replace the calendar item.

Meetings must be Accepted or Declined, never deleted.  If a user is invited to a meeting, and deletes it rather than Decline the invite, CRA will add the meeting back to the user's calendar.  

https://blogs.technet.microsoft.com/exchange/2013/01/17/exchange-2010-calendar-repair-assistant/

Here you can view more details about the Calendar Repair Assistant, the default value for O365 is enabled:
https://learn.microsoft.com/en-us/powershell/module/exchange/set-mailbox?view=exchange-ps#-calendarrepairdisabled

You can use PowerShell (PS) to create a remote session to Office 365 tenant then execute the following commands to disable CalendarRepairAssistant:
Get-Mailbox -ResultSize unlimited | Set-Mailbox -CalendarRepairDisabled $true

This will disable the Calendar Assistant for all mailboxes in the Office 365 tenant.

It can be enabled again by using this PS command:
Get-Mailbox -ResultSize unlimited | Set-Mailbox -CalendarRepairDisabled $false

You can also do this for a single mailbox:
Set-Mailbox -Identity <mailbox> -CalendarRepairDisabled $false.