When you move a mailbox from the on-premises Exchange environment to Exchange Online in a hybrid deployment, the retention policy that's applied to the mailbox no longer works.
You experience this issue even though you took one or both of the following actions:
You re-created the retention policy and assigned it to the mailbox.
You re-created the retention tag and added it to the retention policy.
Retention hold is enabled on the mailbox. By default, retention hold is enabled when a mailbox is moved from the on-premises organization to Exchange Online. When a mailbox is placed on retention hold, the processing of the retention policy for that mailbox is suspended.
To verify that retention hold is enabled on a mailbox, follow these steps: