When viewing the MicrosoftCloudPlugins.log.nptlog there may be lots of warnings as follows:
Message: Exception resolving actor, using event userid username@domain.com, exception: System.AggregateException: One or more errors occurred. ---> Microsoft.Graph.ServiceException: Code: Request_ResourceNotFound
Message: Resource 'username@domain.com' does not exist or one of its queried reference-property objects are not present.