So, me and my friend were doing a Vanaduke run. After finally getting to him, he just DISAPPEARED into thin air. We were stuck in the throne room (and still are as of this post; it's been 1.5 hours) and out of boredom, I decided to do a little test.
First thing's first, it occured to me that the higher level your Glacius/Combuster/Acheron is, the better chance the charge attack has at causing 3 explosions. Nonetheless, it bothered me as to why only some charge attacks would cause 3... So I decided to do 100 charge attacks in a row (I was bored...). Out of that 100, 64 charge attacks caused 3 explosions (yeah... I ticked it down on paper...).
So, it seems we have some sort of a number now to this dilema. Basically, you have a slightly better-than-50%-chance of doing 3 explosions with a level 10 Glacius/Acheron/Combuster.
:D And I'm posting this cause I have nothing else to do while being stuck in Vanaduke's throneroom.
You should submit a ticket (or bug report?) to get your duke back :P