Ahh, I recall that one now.

That's more a balancing issue than a bug - IIRC what happened there was
that the multiplier (basically % of max monster life) used to determine
how much damage an FE explosion causes is not as well balanced on
Nightmare as it is on Normal or Hell.

In fact, it used to be even worse on Hell, but it got rebalanced during
the 1.10 beta ... NM wasn't balanced as well or at all though, possibly
because it depends quite a lot on the max HP of the particular boss
concerned. Indeed it is not so noticeable with bosses that don't have
much HP, unless the other behavior (interaction between multiple *E
mods, and *that*, I believe, is much more likely to really be a bug,
though 1.11 did not to my knowledge fix it) is involved. Else you would
constantly hear about people dying to the Countess's death, or to the
Grand Vizier's or Colenzo's during Chaos or Baal runs, while this
doesn't seem to normally be the case; it seems to almost always be
high-HP enemies like Council, Urdars, Venom Lords, Act 5 minotaurs, and
probably Lister when he gets that mod where people post of losing a
hardcore character.

In fact the NM behavior may actually be more consistent with the
original intent of the Fire Enchanted mod ... by comparison it actually
seems somewhat wimpy on Hell after the rebalance, unless again the
multiple-enchantment bug is involved. But it does seem to be the general
opinion that it's a tad too powerful with high-HP bosses ...