In Investigate / O365 Securlet - they were still missing:
Objects:
Email Message
Email File Attachments
Activities
Send
Receive
The tenant processing was disabled as they had not reactivated O365 Securlet to enable Graph API after repeated reminders to do so before 30 June 2023
Engineering was notified, enabled processing from backend for this client who then deactivated/reactivated O365 Securlet again
After this Mail Objects and Send/Receive Activities began flowing into CloudSOC again