Regression on related entity custom

@colemanw I've hit another 5.41 apiv4 issue on our unit tests. I think a reasonable case should be made that it never should have worked - but it did

On 5.39 this api call

$contributions = Contribution::get(FALSE)->addWhere('trxn_id', 'LIKE', 'Fidelity%')
      ->setSelect(['contact_id.first_name', 'contact_id.last_name', 'Partner.Partner', 'total_amount',
        'contact_id.prefix_id:label', 'contact_id.organization_name', 'contact_id.addressee_display', 'contact_id.addressee_custom'])
      ->addOrderBy('id')
      ->execute();

Was returning the value in the custom data field represented by 'Partner.Partner' - in 5.41 it doesn't.

The 5.41 behaviour does make sense because it feels like the 'right' syntax for 'Partner.Partner' would be contact_id.Partner.Partner but I feel like we need some sort of bridge.

I've hit this in a test but it seems reasonable to think this would not be the one & only place where this is an issue