How badly burned? We talking coals or just browned?
Here is my hypothesis.
Items in boiling water cannot burn. This is due to the fact that the water will consume most of the heat as it changes into gases.
Your hop sac was probably taking somewhat of a direct hit from the burner though. I guess at that point the heat for the water must pass through the hops.
However, hops utilization increases at higher temperatures I think.
This is a question for byo wizard. Lol.
I am sorry I am full of it. Lol.