Is this a bug: Lightning Blast proc by Mana Guide hitting the ground sometimes even if there's an enemy nearby

I circle around an enemy very closely, channelling Focus with Mana Guide. 6-7 out of 10 times Lightning Blast will target the enemy but 3-4 times it just target random location, hitting the ground, even though my character and my cursor are so close to the enemy.
Is this a bug or not?