This is because SGN-HKG is operated under a JSA (Joint Services Agreement) between CX and VN and VN obviously is not a OW carrier. I think OW Explorer fares are only allowed to have revenue shared between alliance carriers (unlike Global Explorer fares) and obviously the VN/CX JSA would mean that revenue on HKG-SGN flights would get split between alliance and non-alliance carriers which is against their agreement.

Other OW JSA routes include the LON-BCN/MAD between BA and IB and MEL/SYD-LHR(via SIN, BKK and HKG) between QF and BA. I think there might even be a JSA between LA and AA but I cannot remember any details. Obviously all these JSAs are between OW carriers so there are no issues.