Auto-casting also seems to be buggy on this spell. Occasionally the auto-cast does not trigger after 1 button press (seemingly when you try to initiate auto-casting from the second or third link in the spell-chain).
When auto-cast does successfully trigger, the period between casts is much longer than if you were to repeatedly press the button yourself. The rate of fire is slow enough to render the auto-casting feature useless for this spell.