I have a similar problem. I want to fly IST-SIN-MDC using SQ and Scoot. There are no actual SQ flights to Manado, only on Scoot and it should be 25k in Y. But even though the redeem page on SQ says it is "Singapore Airlines Group Fly to more places around the world with the Singapore Airlines Group, which includes Singapore Airlines and Scoot." no flights come up on Scoot.

And there is the problem of needing to connect on same ticket in SIN.