dogloverjb as you have experienced QR will interline baggage across reservations with non OW airlines (QR-UA). WorldLux confirms that QR has an interline agreement with SQ so there should be no problem provided the check-in staff are up to it! So it could come down to where you check-in staff at some airports are better trained than others!

If you are checked through and have all boarding passes but your baggage is not checked through you can ask the transfer desk to retrieve and redirect your baggage to your final destination. Not all airports will do this and some (DXB) charge you for it!