From the Diablo 2 Wiki :
Upon death, the fire enchantment will cast Corpse Explosion, causing 75% to 100% of the Unique's max hit points as damage in a radius of 4 yards. This last bonus will be virtually impossible to avoid for those who are close enough to be affected, because the explosion happens barely half a second after the monster's death. Thus, it is essential for melee fighters to maintain a high health level before dealing the finishing blow, or risk dying along with the monster.
In addition, in Nightmare and Hell, minions gain 33% to 50% Fire Damage. In hell, uniques with this bonus also receive immunity to fire.
I researched the thing and found different points of view.
Some players tend to say it is indeed bugged (like in this 2012 D3 forum topic).
Other players tend to say it's working as intended (as written in this Reddit post)
I also remember hearing about this "bug" from an AGDQ speedrun of the game.
Unfortunately, I didn't found any official answer from Blizzard.
EDIT : Still researching, I've stumbled on this video of the aforementionned speedrunner trying to explain the bug.