Negative Hearts?

So, I and my brother found this cool looking floating island and decided to live there. Going up and down took time though, so he got an idea. He wanted me to stand beneath the island, below the edge. He wanted me to hit him as he fell, as a faster way down, with less damage taken. So, it worked - But he got negative hearts. His hearts were gone on the left side, but they were there at the right, and on top of that, he got extra gold hearts. We tried to recreate it and we were successful. We found that the amount of hearts you receive depends on the distance between the ground and the place you fall from. You are able to get even more hearts every time you do it. After, he said he didn't want to play because he felt as if the game was ruined. We were on bedrock. I was working on a behavior pack, which was in fact enabled on this world, but that couldn't have had anything to do with it, because all the pack changed was the size of mobs, and it allowed you to sit on and leash other players, ride any mob, and I also tweaked ender pearls to go straight forward at a velocity of 999, so you could teleport anywhere in sight, so what went wrong? _I recorded it, but the file size is 26 MB over the limit_