It occurred to me that I should watch this FG play frame-by-frame, and then compare it to an XP attempt the same kicker made earlier in the game and watch it frame-by-frame too.
Let's watch one that he made first:https://glb.warriorgeneral.com/game/replay.pl?game_id=3062222&pbp_id=481328
Pause it at the beginning, and then advance it frame by frame. The ball goes "over" the crossbar and continues to spin end-over-end like it's still flying through the air. Then the play continues for three more frames as the ball moves pass the crossbar to the back of the painted area behind the end zone. That's when the ball's spinning animation stops and the ball gets smaller, showing that it's hit the ground behind the end zone.
Now, compare it to the missed FG play at the end:https://glb.warriorgeneral.com/game/replay.pl?game_id=3062222&pbp_id=481575
Again, pause it at the beginning, and then advance it frame by frame. The ball goes through the end zone with the end-over-end animation, but when it gets to the back of the end zone, the ball's spinning animation stops and the ball gets smaller, showing that it's hit the ground right at the back of the end zone.
So, the answer to the question of "What the heck happened on this play?" is "A Level-79 kicker came up short on a 21-yard FG attempt."
Exceedingly rare? Yep. I don't think I've ever seen it before.
Impossible? Nope. Impossible and rare are not the same thing. And just because something's never happened before doesn't mean it can't ever happen. For example, nobody had ever blown a 3-0 lead in a best-of-seven series... until the Yankees blew a 3-0 series lead against the Red Sox in 2005. A 1-seed had never lost to a 16-seed in March Madness... until 1-seed Virginia lost to a 16-seed in 2018.