That's what I thought too, but few months ago I flew LAS-YVR via YYC and did eUpgrade on both segments. It shows 6 credit for my second segment until I hit confirm, then it changed into 0 credit.
Today I came across point 2 of T&C -> aircanada.com/ca/en/aco/home/altitude/eupgrades/terms-and-conditions.html
eUpgrade Credits are calculated based the requested eUpgrade cabin (Business Class, Premium Economy or Premium Rouge) on one-way travel from origin to destination on a single reservation, including connections.
which explains why I get 0 credit on my last LAS trip.
Or was this a gift from AC IT gitch?