SQ, like just about every other legacy carrier, prices its tickets based on your origin and destination, not by individual segment. I’m simplifying it somewhat, but that’s the general principle.

In your case, you’re not purchasing a DPS-SIN + SIN-BKK ticket, but DPS-BKK. It’ll be priced as a through J fare even if you’re sitting in Y for part of the journey.

If you find that booking each segment in your desired class works out cheaper, think about the lack of IRROPS/schedule change protection, then decide if it’s really worthwhile.