Answer
Look for the parent/owning entity that the sub-entity is actually built from — in this case, adding the custom field to BusObjRepresentative (the underlying LU actually used by that Sales Quotation tab) rather than the sub-entity itself works, once you also enable the relevant additional view (here, REPRESENTATIVE_ORD_QUOT) on that entity's configuration. Once that additional view is enabled, the new custom field becomes available to add to the tab through Page Designer, even though it was configured on the parent entity rather than the sub-entity that's actually shown in the URL/page context.