I once had a similar problem with a OW RTW. Had AAA-xBBB-CCC in the middle of a ticket.
Flew AAA-BBB. When i tried to check in at BBB for the CCC flight they noticed the X in the corresponding coupon (it was a paper ticket back then) and after a long wait and numerous phone calls they first wanted to reissue the ticket, but then did non know how and at the end i just had to pay the extra taxes. This was on LAN.
I assume now with electronic tickets it is much easier for the software to spot the X for the SIN-NRT coupon, but no personal experience here, also no experience when the coupon in question is the second coupon.