I was using an in-line eight.

I think the same constricting issue would apply with a butterfly knot. I think it's a function of the type of cord that Hennessey uses. It has Vectran (or Spectra?) filaments as a core. When those filaments are constricted at a point, the properties of the material are somehow compromised, and the cord breaks at the point of the knot.

I did get a dozen or more uses before the line snapped. Perhaps varying the point at which the knot was tied would prevent the snap at the knot point, but I'm a little hesitant to try it. It takes money to replace if it snaps, and this last time it snapped I fell in such a way that I was not injured. Next time? Who knows ...

I think it's just generally a bad idea to tie a knot, irrespective of the type of knot, in the cords used by Hennessy which is a shame because they do allow one to properly tension the hammock.