Yes they are UK citizens flying from Heathrow to Calgary. They have flown with this combination of Passports and eTA before and everything is still in date.

The check in wants a permanent residence card number, no other choices and you can't move forward without it.

They've tried at their end, I've tried at my end, various browsers cookies deleted etc. As soon as you enter a booking ref all personal info; name, DOB, nationality, passport number etc comes up automatically and correctly ( supplied at time of booking) but there is no way to bypass/skip the permanent residence question page.

Air Canada "helpline" was useless just confirmed booking and advised wait till at airport. I will know tomorrow how much chaos this will cause at Heathrow as clearly they are not only ones.