This is not an issue of our tanks not being able to keep aggro enough, at least not the way it has been stated. Both tanks are highly geared and hardly ever miss a taunt or mess up their rotation. But with popping inspiration right in the beginning of the fight to coincide with the power use relics and my Flyby, tanks aren't given enough ability to sustain threat through that kinda of dps output, especially with 2 sentinels hitting master strike during that phase. Threat generation simply hasn't been balanced well to keep up with Dread Guard level gear for DPS.
That being said, we had to wait to use Inspiration and wait to use the power use relics until tanks could generate enough threat. There should be absolutely no concept of "too much dps." This should be something that's impossible to do, especially with a boss that has an enrage timer. The Zorn and Toth fight is way worse, I agree, but the Writhing Horror fight is bugged when you do too much damage too quickly and rip aggro off the tank.
If your start off a fight with Inspiration Bloodlust, Adnreal and relic, then your tank should be using DPS relics (2 x power) , Adrenal, and DPS gears. The alternative is DON'T BE STUPID and wait after the first tank swap.
Ripping aggro off the tank is not a bug. It's a feature of the encounter.