Was a bit irritated with ISM Singapore Bangkok
 

If you weren't flying F/J and only status pax, then I do have bad news for you: Only priority line for F/J class passengers at BKK nowadays, no more options for high status FF. Even, IF they would hand you a priority pass, you'll have to show the boarding pass and, when not in F/J, your admission to the priority line is denied, despite having such a priority card.

The rude DYKWIA passenger got his/her priority card, but no access to the priority line.

Note: My own experience shows, around 3 of 4 times, the priority card with boarding pass is checked (always on departure, less often at arrival), when entering the priority line.
My impression is, when it's "quiet" and the priority line is not relevant (and sometimes "slower" than the normal queues), there is no checking.

So, the ISM was (probably) right, despite you did not like the message.......

I do not have enough data points as mostly i fly J out of bangkok but i flew 4 days ago in Economy and i used fast lane card and the officer check my boarding pass and let me through the lane
It might just be a luck that the officer was just being too lenient or they have fixed the requirements

Perhaps a valid data point:
Flying MH in Business to BKK 2 weeks ago I also asked about the fast lane card.
In MH they are only given at the door when exiting the plane to J passengers as ground crew controls them. They do not carry them on board anymore.

Emeralds in Y should not qualify according to the fast lane rules. There are examples that both the invite and the boarding pass needs to be presented and are checked. (Y passengers with an invite have been rejected). But then when I went through they did not bother with the boarding pass check.

Globalist

I think B/W policy of CX is only passengers in Premium Cabins are entitled to receive a fast track. In some cases, I think some ISMs make an exception but only for DMs without a companion.

I wonder if the ISM arranged the fast track card for the DYKWIA pax knowing that he/she will not be able to use it. That way the ISM has done the job and the DYKWIA will be the custom's problem when he/she hits the line.

From what I know, the new rule in BKK is all Fast Track card will be given by the BKK ground staff upon opening of the aircraft door. Number of cards given are usually exact with 1 or 2 spares.
You may try your luck using the Fast Track card with your non premium class ticket as some times, the staff will check and will reject you. Some DMs i heard from was turned away at the premium lane.

The new rule applies to all airlines, not only CX.

Same with AY. They hand you the blue slip once you leave the plane.

Out of Bangkok? Is your question about entering or leaving Thailand? For flight out of Bangkok, I believe fast lane is also used by Y passengers who are disabled, children, elderly, etc etc. so they probably just assume you're a Y disabled. It's different for entering Thailand.

On my last half dozen or so entries since 'Fast Track' was implemented I've never had to show the boarding pass - the gate-keeper simply collects the card and waves you on.
This includes one time arriving as OWE in 'Y' with CX when the ISM directed me to disembark immediately behind the J pax and pick up the card on the bridge.

Edit: They do check the boarding pass on departure, but they've been doing that for years. Always assumed it was just matching the name with the passport, but maybe they're checking class of service too?

Leaving bangkok...but yesterday i just came back in Y but alone the ISM asked me to go to the front and gave me the fast track at the exit. And I was able to use it. So it seems it is ok now

yes, i had experienced myself. Arriving in Y but had fast track and it was ok to gp through but it may depend on the ISM if they let DM go to the front during the deplane and hand the card at the exit. However i assumed when i travelled alone, they would comply but if with family, they would give an excuse.as they might not have lots of extra