My experience was different than your's. I was flying business class into BKK on my first flight after reaching 50K, so I already had lounge access, but out of curiosity asked the check-in attendant about the situation. She said she could see my last flight, and see how many miles I had, but couldn't check to see that my last flight was to be credited to ROP or not, so thus couldn't verify if I had gold status or not. I had my previous boarding pass with me, so I could have shown her that to prove that the previous flight was indeed credited to ROP, but it didn't really matter in my case. She told me to contact ROP and ask for the letter verifying I was Gold if I was going to fly any in the next month after that. So I think it comes down to the discretion of the check-in / lounge staff in this case. If you're at BKK, it doesn't cost them anything to let you in the lounge, but if you're at an airport that doesn't have a TG lounge, it will cost them and so I'd guess they'd be less inclined to allow you in without proof.