Killing enemies on first tick sometimes spreads, sometimes doesn’t (seem to) spread. I have manually applied BP to individual maggots that spawned from the FG mummy enemies (which with the old version got killed almost instantly because of the mummy corpse), but they just die and don’t spread BP. Sometimes small packs do blow up. Not sure why (could also be other procs), but seems that the death spread radius is small.
Build used in the video: Occultist, Level 43 (GD 1.2.1.3) - Grim Dawn Build Calculator
EDIT: Added video and build for reference.