They'll deagro with DoT on, and if it's time based then it's time since agro and not time since last action.
On Ice/Thundersday I used to do something like:
Pull a Prince with T4, Grav, Run back, B4, T4, Bio II, Drain, Bind -> /heal to get a tick or two before it dies for Chain 3.
And I had them despawn on me like that, remarkably quickly too. They'd deagro while still gravitied, bound, and DoTed.
It's like they just hit certain windows since the first action/their last hit/something I don't know and decide to deagro.
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