Their website said “Bangkok”. I’ve traveled all over Asia and Europe without this problem.
There are a lot of things to complain about when it comes to airlines in general, and AirAsia specifically. But not knowing what airport you booked, that's on you.
If you Google for “Bangkok to Singapore” half of the flights Google will show you are from DMK.
If they’d listed UTP as being Bangkok, then sure, I’d have some sympathy, but referring to DMK as being Bangkok is probably more valid than referring to NEW as New York.