Core/Spells: Healing spells with SPELL_ATTR6_NO_DONE_PCT_DAMAGE_MODS get...

... no healing taken mods (confirmed for Leeching Swarm)

http://www.worldoflogs.com/reports/jz78urnmw3a3a01p/details/213/?s=6803&e=7037 a raid with a rogue using Wound Poison, and there are as well many suspicions on various forums