Per below, I have Canvas configured to do a 50% deduction on late assignments.
The problem is this is not a 50% deduction. Instead, it appears to be deducting 50 points. For example, if I have a 25-point assignment, the student gets a 0 for a late assignment.
Even if the interval is the issue, it would take many days before successive 50% deductions result in a score that rounds to 0. I am well within such a period.