The bug

The effect radius of a sweeping attack is a sphere with a radius of about 1.5 blocks. This area usually overlaps with the player's hitbox mimicing the act of sweeping with a sword. However, with an increased attack reach, the sphere moves to the entity the player hits, detaching itself from the player, making it possible to hit multiple enemies from far away while not hitting the enemy that is actually hitting them. 

These videos show how wierd the sweeping edge behavior is. 

My test on the 1st snapshot: https://www.youtube.com/watch?v=BnR-bbFvCqE&t=8s

My test on the 2nd snapshot: https://www.youtube.com/watch?v=wPe0NYQTZho&feature=youtu.be