...It seems there is no problem when you search the entire journey with return - but when doing them separately the avails change (when using sample date).....
This is normal -- PER-YVR will be available when HKG-YVR is not on the same plane! Cause is the yield optimization that all airlines do now, in O/D traffic. Unfortunately this also makes a HKG stopover difficult (but not impossible). You could try TPE-YVR or MNL-YVR which might be available when HKG-YVR is not (then you'd need a separate ticket to position HKG-TPE or MNL). Not really worth the effort. Note that J award availability to YVR is notoriously difficult to get, hence the advice to move fast if you see availability