Piercing Arrows
It’s because of how bows work in this game, when you aim at the closest target you will aim for the arrow to hit in the chest on them (which is why bows lead running targets).
Due to this it doesn’t take into account that you also want to hit the target behind.
You can see it best if you’re fighting something like giants as you will actually shoot up into their chest.
It is ‘working as intended’ so to speak, it’s just poorly designed when piercing arrows come into play.
As Jaycat explained, it’s a matter of angles. The piercing always works, but since you’re always shooting at your enemy’s chest, if their height is different from yours, especially with shorter enemies (asura risen….), you end up shooting into the ground or sky. That’s why it’s usually better to target the backline and line up the shot. Cause you guarantee that it’ll hit everyone else in the way.