It's funny - I had the opposite experience. We were flying LAX-AKL in J and I had read here about the TBIT F lounge being for F only, so I headed to the J lounge on the 4th floor. When I got there, I was told that being Platinum I should go up to the F lounge on the 5th floor, where we were gladly welcomed. I guess the solution is to try and avoid TBIT & leave on flights from T4 - have never, ever had a problem using the flagship lounge there!