Currently the Amend Mailbox Permissions form does not load organizations at the child level. It appears to be the only Microsoft Permissions crate like that, whereas all of the others do run from the child level. See Enable/Disable Mailbox Forwarding, Configure Out of Office on Mailbox, Amend Calendar Permission on User, Reset Microsoft MFA, and Add or Remove Group Membership, for examples of Microsoft Crates that can be run from the child organization level.