I'm QFF Plat flying today from Syd-Seoul on Asiana, booked with a QF flight number.

F alternative I meant

That has happened to me as well but only once. I am a QFF Plat and was flying SYD-CAN on CZ with a QF ticket. I was refused entry at both the First & Business lounges because  the flight was opperated by CZ. They directed me to the Skyteam lounge insted.  

I recently flew SYD - PVG again on a QF ticket but opperated by MU and was allowed access into the QF F lounge regardless that I was flying on MU. 

One could argue the F lounge in SYD is always at capacity - certainly seems that way each time I visit mid-morning before all the major flight departures!!!

Thanks for the response, to clarify I was booked on an economy ticket, have flown this route before with F lounge access and was told at check in it was available. Qantas website states Plat on a  QF marketed flight is eligible for F lounge access https://www.qantas.com/fflyer/dyn/flying/tier-benefits#benefits-of-platinum ... No big deal was just annoyed at the inconsistency and wondered how often it happened...