You can’t show your J or F boarding pass if you’re travelling in Y. (note: yes, I know)
Previously diamonds and emeralds would get a fast track pass when departing BKK even if in Y.
Well tough darts for them now. It's a J/F (plus monks) facility that has been abused for years by people AOT said were ineligible (e.g. not in J or F). Usually by squeezing a pass out of aircrew since the agents guarding the entrance to Fast Track would need an encyclopedic knowledge of what levels are what on 100 airlines. Then the airlines of recent started giving them out only on landing to people in the front cabin.
Blame the fast track agencies for bollocking it up maybe. The fact they had to change the cards 3 or 4 times in under a year clearly showed people were up to hijinks.