Has an AFL game ever ended 31-28?
Exactly once. On August 29, 1903, Carlton beat St Kilda 31-28, and no AFL game before or since has ended that way. When it happened it was a scorigami, the first 31-28 in league history.
That makes 31-28 the 3836th most common final score out of 6,607 that have ever occurred in AFL. One point away, 32-28 has happened never and 31-29 has happened 3 times.