Annoying Sion E Bug

If I am not mistaken, Sion's E works like this: When you roar at an enemy minion, they fly backwards (and cannot die while flying), hitting additional targets for 150% damage and slowing them too until the they have either gone a certain distance or hit terrain.

I play a lot of top Sion and what has been bothering me is this is only true about half the time. Sometimes it works as intended and is a really rewarding spell in terms of damage and CC, but other times... The minion will sometimes completely fail to fly and just get stuck either immediately or halfway through and hit some sort of invisible wall. Other times the minion will somehow die during the knockback (Sunfire Cape?), even though it should be impossible except with smite to kill them before they get to the end.

I really enjoy playing Sion but this bug has cost me quite a few trades toplane as it is supposed to be his most reliable damage.