Originally Posted by
immunedata
If this was booked as a single PNR connecting flight then OneWorld rules state if the F flight was the long haul flight then you would indeed be eligible for the F lounge on departure of the short haul segment. OP's problem is that he doesn't (strictly speaking) have a connecting flight.
There is no such requirement for both segments to be on the same PNR. The real issue here is that the MH lounge staff tend not to be aware of this rule, and even an escalation won’t get you anywhere.