I routinely return to Canada from the US and Europe without any problem getting BPs doing OLCI. During OLCI I was never asked to provide my PR card. In case of documents check, it occurred at gate, and I never showed my PR card: they always checked my passport.

My theory is that either AC can get access to confirmation of my PR status by scanning my passport or they have linked my PR card to my passport in the past (probably manually when I checked in at the counter.