Technically, CBSA is required to ensure that each traveler (PR landing or otherwise) has reasonable plans for quarantining. CBSA does not actually enforce quarantine, they only review your plans. If the CBSA officer applies some common sense it is easy to see that turning around and returning to US is the best quarantine possible with zero risk to any Canadian residents. Some officers will follow this logic and allow you to complete your landing/entry. Some will not. It is highly subjective and will vary by person.