Am trying to remove full mailbox delegation for a mailbox
Have you tried removing it via PowerShell, use the Remove-MailboxPermission cmdlet. If the mailbox is located in ExO, you can also "reset" its permissions, or clear Automapping: https://www.michev.info/Blog/Post/1382/removing-automapping-and-resetting-default-mailbox-permissions-in-exchange-online
Use the Remove-MailboxPermission cmdlet to remove permissions from a user's mailbox or from an Exchange Server 2016, Exchange Server 2019, or Exchange Online mail user.
For information about the parameter sets in the Syntax section below link,
You need to be assigned permissions before you can run this cmdlet. Although this topic lists all parameters for the cmdlet, you may not have access to some parameters if they're not included in the permissions assigned to you.
Follow the link below for further assistance,
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