Yes, in the past, I have been able to collect the reward. I've had the balrog achievement for some time.
I just played through the elite enemies challenge. At the end of dungeon 4, I was able to kill a single balrog (it was the only creature left) and collect the experience. At the end of level 5, two balrogs remained. A single Rain of Fire killed both. I was able to collect one, but not the other. When I try to collect the second, a buzzer noise plays. I tried several times, cast Rain of Fire again, etc. No success.
I *think* that I had revealed the collectible balrog with a bloodrose potion, and that the uncollectible balrog was just a square I had marked 300. So that may be worth checking.
I checked the enemy list scroll and it showed x1 balrog. This screenshot shows the uncollectible one
Here's a list of what I was carrying at the time of the error:
2 bloodrose potions, 1 rain of fire, 1 dice, 1 nature's sight, 1 bounty
2 lunar pin, 1 copper coin, 1 credit card, 1 vampire fangs, 1 egg, 5 gold coin, 1 fairie bait, 1 topaz amulet, 1 claymancy, 1 superior flute, 1 lich detector, 1 abacus, 3 silver coin, 1 reroll card, 2 chest detector, 1 gloves of healing, 1 treasure map, 1 candle, 1 bat bait, 1 meat grinder, 1 rat detector, 1 wishing well, 1 brick detector, 2 lantern, 1 crystal ball, 1 campfire, 1 pickaxe, 1 spiritual tattoos