Exchange Online organizations configured with multiple geos (satellite regions) still find it problematic to search for mailbox audit events. Microsoft might have fixed the problem for admin audit events, but mailbox audit events are often more important.
In March 2020, I wrote about mailbox audit events for Office 365 E3 accounts not showing up in the Office 365 audit log. As far as I can tell, Exchange Online deals with new mailboxes properly now. However, there might be some mailboxes in your organization that aren’t generating the audit records you thought they are… so it’s time to check.