I'm not sure about CX839/838, but I do know for 888/889 all sectors (HKG-YVR-JFK-YVR-HKG) are very frequently staffed by HK-based crew. I know I've said this before apologies for repeating it. But I do think sometimes people get mistaken by thinking flights ex a certain destination must be a certain crew base.

888/889 are available to be rostered (and swapped) for HK-based staff. 888/889 is the longest (and highest earning) pattern (I believe 7 or 8 days) for HK-based cabin crew, and many of them actively swap for it due to this reason. If I recall the first layover is in YVR for a few days after doing HKG-YVR, then HKG-JFK is a turnaround with the return to YVR and another multi-day layover, and finally the return sector is flown YVR-HKG. I do not know how frequently YVR-based staff fly 888/889 but I definitely know that HK-based crew are actively rostered on it.