I often fly the BKK-BOM flight, which is a 747 sold as two-class.
Once asked at check-in for a F class seat, was told it's full.
Another time, asked at check-in and was immediately allocated F class window seat at check-in.
Last week, asked at check-in, told it's full (asked if it's occupied or blocked, and told in a very snappy and surly manner "IT'S FULL!"), but "upgraded" at gate to 2E (which despite being a lone seat in the middle of the cabin, is surprisingly private.
YMMV, but as everything else with THAI, it's consistently inconsistent.
I'm SQ*G, if that helps.