When using a weapon enchanted with Frostbite from the mod "Enchancement" to kill a boss (specifically the Nether Gauntlet is what I tested this on), the boss will turn into an ice sculpture as expected from the enchantment, but its death animation will no longer play, resulting in nothing being dropped.

I opened this as an issue in the Enchancement page and apparently it's because the bosses in this mod aren't properly tagged as bosses.
If they get properly tagged it will fix this issue because Frostbite by default doesn't work on entities tagged as bosses

This should be fixed in 1.7.3