Hi,
Could you please let us know how the mailbox permission has been assigned?
Exchange hybrid deployments support the use of some delegated permissions in on-premise Exchange, but not all.
https://learn.microsoft.com/en-us/exchange/permissions#delegate-mailbox-permissions
In order for this permission to work, you may need to upgrade Azure AD connect to at least version 1.1.553.0. Also Enable Exchange Hybrid to writeback the permissions.
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