OK, but two questions (with the assumptions that the rate rules are that the cancellation must be made by 6PM hotel time on day of arrival and the OP was in compliance of them.)

Scenario 1. I cancel a registration successfully (During the transaction I receive a Cancellation number followed by an email confirming cancellation is received shortly thereafter. Can a front desk associate check-in some one else sometime later in the same day using that now-cancelled reservation as it was made by the original party with their preferences, MR# CC# etc.?

Scenario 2 Involves another issue (Given -you can access a record after date has passed) On the same day, can a cancellation be successfully processed the same day subsequent to a legitimate check-in?

BTW, when front desk associates access a record I assume it's a name look-up (I'm never asked for my reservation #). After all, it is possible to confuse two different guests who happen to share a common name.

Correct me if I'm wrong, but if a reservation is cancelled in compliance with the Cancellation Policy included under the rate rules both on line and contained in confirmation of reservation emails there shouldn't be an issue insofar as the OP is concerned, correct? And if that's true, the checking-in of someone else, whatever their names happens to be, (erroniously) under a reservation canceled in compliance with those rules should not be the OP's concern nor his liability, should it? (it does appear to be some kind of systems hiccup.)