Caster Mastery 25% Chance to Open Chests bug in Delve

There is an encounter called "Howling Den" in the "Freezing Hollow" biome.

In this encounter there are four piles of meat you must poison, which are in the darkness.

There are two chests provided containing flares to use to light the darkness to get to the meat.

If you manually click the chest to open it, you are awarded some flares.

If you open the chests with Caster Mastery "25% Chance to Open Nearby Chests When You Cast a Spell", you do not get any flares but the chests are opened and wasted.