When I checked in at Paddington on Friday for LHR-YYZ-YOW (CP087+CP1582, note no codeshare), the baggage tag was for YOW, but the BA agent couldn't get a boarding pass to print for YYZ-YOW.
I told her not to worry about it, I had to deal with a CP agent at the transfer desk in Toronto anyway (after clearing customs). It did seem strange.
Re: priority tags: I've found CP (and AA) are useless at dealing with those red priority tags. On my first trip YOW-YVR-LAX on CP, my bag was literally the _last_ one onto the carousel at LAX -- the only thing after it was the plastic bin saying "no more bags". I travel without checked luggage now. On my LHR trip just completed, my bag was consistently somewhere in the middle each time, and always after some non-priority tagged bags came out.
FWIW, AC's record was better than 80% at having my priority bag come out in the first wave before any non-priority (at YYZ going both ways, at LAX, and at YOW).
Maybe now that they've taken over, they can explain to CP handlers what "PRIORITY" means.
andrew