No worries.

It is a VERY common misconception that I hope everyone learns as it gives you a lot of flexibility.

Re: the TG F lounge at BKK, I have used it when flying outbound on TG, SQ, UA and LH. No problem.

N.B., I have also used it when flying TG in C to HKG, connecting same day to SQ in F.

While the lounge is available, some services apparently aren't (the spa). Frankly, I don't get to the airport all that early to use spa services anyway.

Regarding this class of service lounge access rule, keep in mind that the lounge must be actually operated by a Star carrier (e.g., if a Star carrier uses a lounge operated by Aeroflot in Moscow and you are flying another Star carrier, you may very well be denied access).

The only airline violating this lounge access rule is LH, which will not allow you into the HON/First lounges in FRA and MUC and the HON/First Class Terminal at FRA. Those lounges can only be used by LH (and its affiliates, e.g., Swiss Air). They will instead send you to the grossly inferior Senator Lounges, which they call first class lounges, but they really mean first class for every Star airline passenger except LH.