This happened a while ago, on the Mother Adria quest. There was a negative energy trap on the ground, and someone summoned a ghost to trigger it. The ghost was killed by the negative energy.

Shouldn't undead be healed/buffed by negative traps?

It's a Bioware issue, far as I can tell. Might even be hard-coded!

Negative Plane Avatar is harmed by Inflict Wounds as well.