Seems incomptetent HR making issues out of the blue. I am wondering if they say to a Canadian citizen "your passport/driving licence expired, we cannot be sure you're citizen..."It states that: "unfortunately we won’t be able to have you come onboard with us until you demonstrate you are eligible to work in Canada with your obtained PR status."
As others said "PR status" is way different from PR card. Just show them the signed CoPR, or the expired PR card. SIN number is also excellent indication.
If none of this suffices, you could *try* to have an immigration lawyer writing an letter explaining that an expired PR card does not remove PR status (conditional to RO)