I have just experienced this problem only a couple of days ago.

I arrived at BKK at 1.00pm & departed the next morning at 8am for BNE. Thai paid for an overnight stay at the Novotel airport hotel. When I arrived, I asked a Thai customer service person if I had to pay a departure tax the next morning. She advised me that I did not have to as it was included in the ticket price.

When I checked in the next morning, I was told I had to pay the departure tax. This created an unpleasant experience as I argued that I had been advised otherwise & I did not have any baht. I ended up having to go back out & exchange for the necessary baht.

This spoilt an otherwise very pleasant flight with Thai.