Im hesitant to post this question as I'm sure a definitve answer must have been documented here but I haven't found it after doing a search for "mixed" through the 1500+ posts in this thread. So if I missed it, please don't flame me!

I see posts from a few years ago that state the answer is sometimes agent dependant.

My scenario:
Traveling on a Business Class Lowest ticket YYZ-YVR-SEA. YYZ-YVR is in J on 789. Connecting flight YVR-SEA is economy only Jazz. I have a 4.5 hour layover in YVR (hopefully I can SDC to shorten that a bit). Coming from the Delta world where lounge access is granted on the entire direction of the trip, that logic would imply YVR-SEA still gets me lounge access. But there are some posts claiming that its passed ONLY on your next outbound flight from the city you are in. Surely that makes no sense when the last leg is on an all-Y aircraft and you were in Signature Class across the country? Has AC not clarified this scenario or am I up the the mood of the lounge dragon on travel day?

Footnote: I have no status on AC or Star Alliance.