I was able to book the OP’s itinerary last summer same price ($1800-$1900) though I bumped it to PremPlus for $2400 RT (Plus Points to Polaris cleared one way so I was happy enough). The SQ segments were issued in Q fare bucket. All on one 016 ticket.

OP - last summer there were two SQ flights, 8pm and 10:50pm departure out of SIN to MAA. See if the later flight is still there - for me that was the cheaper one.

Before last summer I have bought UA SFO-SIN / SIN-India as two tickets. At least 10 times - usually connect to SQ, rarely AI and once I think Jet. Just need to give extra time in SFO for a savvy agent to through-check the bags all the way.