In September, an AC pilot (of a cargo flight) tested positive for Covid after landing in Hong Kong - and the entire 4 person pilot crew were detained as a result.
ref: https://torontosun.com/news/national...ive-covid-test

As a result, Air Canada no longer have any crew stay in (or even "enter") Hong Kong. The stop in Seoul allows the inbound crew from Canada to disembark (and they stay in Seoul). There's another AC crew who board in Seoul - and the Seoul-Hong Kong flight is short enough (about 3 hours) that they can complete the Seoul-Hong Kong-Seoul round trip and return back to Seoul in one day. That crew then disembark (and spend the night+ in Seoul), and a different crew fly the long haul back to Canada.