Exchange Online enables mailbox auditing by default, which should mean that audit events get to the Office 365 audit log for all E3 and E5 mailboxes. Well, that’s what you might thing but that’s not what happens. Mailbox events for E5 mailboxes arrive just fine, but you must reenable E3 mailboxes for auditing before their events flow. It’s a bizarre situation.
https://office365itpros.com/2020/03/12/exchange-online-mailbox-auditing-default-problem/
