It wasn't just your computer showing no availability around Dec 2. My computer showed that too. It also showed no availability for one-way or round trip on EF. If I changed to JFK-HKG, it did show plenty of availability. It is indeed odd. My feeling is that CX may be switching flight time or something.

On your additional update, HKG is usually the HIP. It is strange that YVR is also a HIP, but that may be because of date difference. The week starting from 2-3 days after Thanksgiving is usually a very low travel period. OTOH, you can buy Jan 27 on CX865 returning Jan 29 for $429. I suggest you fly on Jan 27 and discard or change the return. That saves you $274.