Here is my test on my tenant, you could have a compare with yours:
Mailboxes: OUB2, OUB3, External Mailbox.
Configure OUB3 as OUB2 delegate:
External mailbox sends a meeting request to OUB2.
The meeting request show in OUB2:
The meeting request show in OUB3:
OUB3 accepted meeting request for OUB2:
Check from OUB2, this meeting request still shows in Inbox, but we can see it has been accepted by OUB3:
From the external mailbox, we can see OUB2 has accepted this meeting:
This meeting shows in OUB2 calendar, it doesn't show in OUB3 calendar.
Please compare it with yours and tell us which steps are different.
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