Penumbral Blast inconsistently poisons with Necrotic Grips
It looks like the freeze from the melee is cancelling the poison status. No damage over time form the poison once the freeze fully activates.
If I get a final blow with PB, the poison spread works.
If I get a direct hit that is not a final blow, the poison can be seen for a few frames before the freeze kicks in. But the frozen animation removes both the poison animation and the DoT ticks.
If I get an indirect splash hit with PB, the target is not frozen, and takes poison damage until the poison runs out.
Hopefully someone else can replicate/corroborate these issues with the melee interacting with this exotic.
Edit: When I get a PB final blow that spreads poison and creates iceflares, the iceflare bolt that finds a nearby poisoned enemy will freeze and cancel that poison status. This is what lead me to believe the freeze status is conflicting with the poison status.