Hi @GoodResource ,
Does this issue affect the assistant's own calendar?
Do these disappeared items have anything in common?
Based on my experience, I'd suggest checking if the assistant has any calendar related third-party add-ins enabled in Outlook clients. If yes, you can try disabling them and monitor some time to see the result.
Besides, you may consider removing the assistant's permissions and then assigning permissions again to check if there would be any difference.