unattach parts on shipment handling unit

Hi

We sometimes have a need to adjust handling units when the parts are ready to ‘go out the door’, typically splitting the contents of one handling unit into two.  For handling units in stock I can select ‘unattach part’ and can then re-assign that part to a new handling unit.  For shipping handling units though, if I select ‘unattach part’ the list of parts available for un-attaching is always blank.

Selecting ‘Unattach parts’ shows this:

I know we can achieve what we need to by disconnecting the existing handling unit, re-creating two more and assigning the parts, but it would be much quicker if we could just unattach a part from the existing handling unit and then re-assign to the new one.

Is there a particular reason why the list of parts to unattach shows as blank for shipment handling units?

We are using Cloud 22R2

Thanks!