Caster Mastery: 25% chance to open chests bug in labyrinth

A small bug I discovered. When using the caster mastery node which grants "25% chance to open nearby chests when you cast a spell," the chests which give items against Izaro's adds (in this case, the instant Lieutenant killer) doesn't drop the item, but the chest opens anyhow.

For clarity: The chest was opened using the 25% chance given by spellcasting, not by hand. I am also using frostbolt and ice nova combo with spell echo if that helps.