Actually, the problem is the opposite: no problem at the PoE so long as you have a valid passport (to establish identity), but the problem may be in boarding a plane headed for Canada.
As a PR you are entitled to enter Canada. So all a PR needs to do is establish identity and status, and the proof of identity will usually suffice to establish status (makes the entry easier if the PR is carrying anything to show IRCC client number, like the CoPR).
In particular, unless you are a U.S. citizen, or one of the other very small classes of exceptions to eTA, a Canadian PR must present either a valid PR card or a PR Travel Document to get CBSA clearance (via electronic system all airlines with flights to Canada must now use to clear passengers before issuing a boarding pass) to board the flight.
I cannot guess whether or not there is some possibility of slipping through pre-boarding screening by displaying a no-longer valid Canadian visa (any Canadian visa would be rendered no longer valid by landing and becoming a PR, even if the physical visa is not marked as such). For sure, technically should not happen; that is, for sure, in order to board the flight you should have to show PR card or PR TD. Practically, I am guessing that depends on how the Advance Passenger Information system (operated by CBSA and accessed by the airlines) actually functions, which of course is not public information (beyond the fact that it should not give a pass to board for a PR unless the PR presents a valid PR card or PR TD or has an exception to eTA like a U.S. citizen).