I just investigated. My first reaction was that hellfire hasn't been changed in many years and I verified that by looking at the code commit history.
But I figured it out after debugging. It turns out there WAS an announced change which is responsible for the new behavior:
Code Update 12/7/2015 - Fire/cold/lightning damage is not reduced by racial magic resistance
Sprites are resistant to magic which means that prior to this change they effectively had both resist fire and resist negative. Hellfire defaults to negative but switches to fire if it's better. Because both types were resisted, the code stuck with negative energy.
After the code change, sprites no longer have resist fire but still have resist negative. Therefore hellfire selects fire instead of negative. The same is true for deep-elves.
If it's being implied that hellfire is never negative, you can test that's not true by using it against an elf and see that it works as it did before. Elves are resistant to magic but they are also vulnerable to negative energy, so hellfire stays with negative.
I hadn't really considered this behavior when I made the change, but it follows logically from the announcement. So you can't call this a stealth nerf.
Note that hellfire has never checked for the presence of the 'resist elements' spell when determining which type to use. It has only ever used innate racial resistance. Should it do so? I don't have a strong opinion, so the community can feel free to weigh in.