I'm seeing exactly opposite of what OP is. With tenacity down applied I barely ever see resists. What I do see, is the Teebo stealth bug sticking it's ugly head out a lot more, and Dengar's slow is very unreliable...
Now I see one of two things happening... Either OP had really bad rng, which could be considering this was his first and only attempt in raid with Dengar, or, they screwed something up with this in the patch that fixed the other bugs with his tenacity down.
I got 3.2 million in my raid yesterday with Teebo 68% potency, QGJ 80% potency, EE, GS, Dengar... So I'll say right now it was working fine yesterday at 10am est.