Winter's Guile is supposed to make targets frozen by Penumbral Blast automatically shatter in addition to its ramping melee damage. This works as intended for the Shadebinder subclass, but is currently not automatically shattering targets when Prismatic is equipped. Video linked shows this issue, using Winter's Guile with both Winter's Wrath and another super, so it doesn't seem to be tied to equipped super. The other part of the exotic, the melee damage, works fine as shown.