AP Invoice hold notification does not send for Non Releasable hold

Summary:

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We need to send the AP invoice hold notification to PO requester and for that we need to enable "Allow Holds Resolution Routing" in hold setup. However, what we found that for non releasable holds this check box is greyed out and can not be enabled and hence notification will not be sent out. Please let me know if this is correct and if yes why it is only for releasable holds and is there any way to send the notification for non releasable holds as well.
Thanks.

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