You talked with AM (part of CX), not AS, I assume? I don't see this rule in the T&C, but to be honest, I never redeemed a oneworld multi-carrier award that doesn't start and end in Taiwan as well, so maybe others can confirm. If you really need to go back to SIN to redeem, you can have an HKG//KUL open-jaw and add KUL-SIN on MH at the end. Only a little bit of extra tax and no difference in the amount of miles required.