Ignore def doesn't stack

It does? I saw no difference in damage with and without it, on Chief Oblivion Guardians. They're normal mobs, right?

I'll reproduce it and take a screencapture, be right back.
Edit: Okay, I don't even know anymore.
Max range: 12859 clean
Max range with no Ignore PDR: 12859 * (1 - 0.1) = 11573
Max range with Ignore PDR 30% (Codex): 12859 * (1 - 0.1*0.7) = 11958
Max range with Ignore PDR 2.5% (Ambition): 12859 * (1 - 0.1*0.975) = 11605

Achieved damage:

 Spoiler

Highest I got in a 5-minute-long whacking streak was 1177x, still a good margin from 11958.. It's somewhere a bit more than what Ambition gives, but nowhere close to what Codex gives, let alone both.