I wanted to update people on the outcome of this case.
The Coffee Club Thailand head office confirmed that there is no service charge when points are redeemed on their app. The 12 Baht charged by the branch on Sukhumvit 11 was a breach of their own terms and conditions.
The manager refunded the 12 baht and made an apology.
Now, this isn't about the money, or the fact that the staff chased after me so it looked like I was trying to leave without paying.
When I returned to dispute the service charge, every one of the staff were insistent that making a service charge for the 'free' drink was correct. This is despite me explaining to them that I had redeemed points in the same branch just a couple of weeks earlier and not been given a service charge. Indeed, I've redeemed points on their app at branches around Bangkok and the rest of the country without ever having been made to pay a service charge for the free loyalty reward drinks.
Unfortunately, cases like this are symptomatic of Thais in almost every walk of life, but particularly in the service industry. When they don't know the answer or they do things incorrectly the simply come up with some fabrication or nonsense explanation (translation = BS) for what they're doing.
The majority of Thais accept what they are told in these circumstances without questioning it. It's always the customer that loses out.
I would implore everyone never to accept the words that are told to them in Thailand question them critically and rationally.