I currently have SIN-EWR in J booked on SQ and was looking to add KUL-SIN (stopover) SQ and EWR-YYZ UA to my itinerary, for the final itinerary to be: KUL-SIN (stopover)-EWR-YYZ:

KUL-SIN SQ J
SIN-EWR SQ J (Stopover)
EWR-YYZ UA Y

I called in to make the change and the following happened:

1. KUL-SIN SQ was able to be added but only Y could be added to the itinerary. The CSR said that on their end no J was available to be added to the itinerary but Y was, despite the route KUL-SIN on SQ showing plentiful J availability at different times when searched separately

2. EWR-YYZ UA was not able to be added at all as the CSR mentioned that the original SIN-EWR SQ segment still must be available in order for a final destination change (end segment) to be added. Originally I asked for the KUL-SIN-EWR-YYZ to be priced, but CSR said that the system couldn't price it, nor could the ticket be issued because it wasn't able to be priced.

Has anyone encountered these issues before? I know adding EWR-YYZ UA Y to SIN-EWR SQ J is certainly doable as I've done it while the SIN-EWR SQ J segment was not available, so am a bit confused as to why it isn't working now. Also confused as to why KUL-SIN J can't be added despite there being a lot of availability..

Is this a HUCA situation or are these the new restrictions imposed on AP segments that are for certain?

Thanks in advance for your help!