I got Ancient Guardian damaged enough that he started doing that jump attack. I died and came back later, and he did the jump attack across a gap in the floor and into the labyrinth and killed me again. Which was fine, my fault for getting so close so carelessly. But then he tried to go back into the arena and got stuck walking/hopping endlessly against one of the permanent pillars, which was in the middle of the only route into the arena. The endless hopping behavior looked like it was unintended, because he was stuck trying to get back inside, so I figured I'd report it.
Get guardian to do the jump attack at you from across a gap between the arena and the labyrinth, lose aggro (probably by dying) and watch him try to hop back into the arena. If there's a pillar in the way, he'll be stuck outside.