I don't see the issue with the events you posted, but you said that those aren't the events that were causing the problem, so that doesn't help much.
Anyway, I've had similar issue, the problem was if you spawn a bullet then change the direction you are fine, but if the bullet gets destroyed before the the angle is set then it picks all the bullets. LOS may be going in and out quickly? not sure, so maybe this doesn't apply.
Anyway, first thing I would try is 'On Bullet Created' -> set angle to: boss.direction = "right" ? 0 : 180
(if you don't know the ?: I recommend learning, very useful)