From the patch notes:

"Plaguebreak: This set now will only attempt to apply itself to enemy players, rather than any enemy. Note that the
explosion can still damage and be amplified by nearby non-players.

Developer Comment:
Spoiler
This change was made to correct a behavior we've been seeing in PvP areas where you could apply this set to entities (such
as pets or guards) that lacked critical knowledge of how to counter-play the mechanics introduced. This also hits the truly
creative uses of the set in PvE where we have seen some humorous videos that wipe out entire dungeons with relative ease,
which while we appreciated the ingenuity, it was *slightly* over performing."

So it won't proc off NPCs or pets (twilight, engine guardian).