I'm wondering the same as well for Plagueburst rune that leaves a glass cloud when enemies die. I'd assume +ailment dmg/passives affect this but not 100% certain.

This is by no means a definitive answer and am still looking for confirmation from a dev or moderator, but I cleared my passives and speced up to the nodes right before the -% dmg to non DoTs. I got mostly ticks of 2200-2350 for my tear and 650-750 for my anomaly skills. I then got the 4 Cabalist nodes (plus the ones in between in order to get all 4) that reduce dmg done, x2 Crippling Decrepitude, Insidious Decay, and Incessant Infirmary. My numbers seemed slightly higher with 2280-2400 on tear and 680-800 on anomaly. I also made sure I hit the same monsters in case of differences in innate resistances. The only node in between that I had to get that added dmg was a single +25 wisdom stat node.

If I had to guess what's happening, I would say the -% dmg does not seem to affect the AoE zone dmg and the slight increase in dmg seems to check out from the +25 wisdom stat bonus. If the -% dmg nodes were coming into affect I would have expected a large and noticable decrease of dmg in the zones, but was not the case.