Since you asked....
Its not a bug but more about balance... I like the essence bricks concept, force you to kill mobs and stuff but... They kinda need a balance!
If you play on hard(I always play on hard), no way you are going outside killing stuff! I just die in 10 seconds. Fishing zombies, nasty spiders and the sudden creeper that was behind the tree, no chance of go hunting! So... you have to rely on mob farms and auto-spawner/grinders. If you follow the quest book and not build stuff ahead of time, you will only be able to rely on mob farms for a long time.
And this is the problem! Mob farms are way inefficient, since its dark outside, every place is a spawnnable place and you dont have torchs to just torch 8 chunk radious around you! And then, you have the caves... 'Torch slabbing' the caves is kinda doable, but slabbing the whole surface is impossible and suicidal!
So... maybe tweak the smeltery recipe to need less mob drops to make the essence? And/or maybe, add zombie awareness or a similar mod that makes mobs be more aggressive on you, so the mob 60 blocks away will try to pathfind you and fall on some cleaver trap making the farming easier.
Currently, if the mob is like 10 blocks away from you(creepers) or if he doesnt have a clean sight of you, he will ignore you. Would love if they were smarter and try to get to my house(hole) and just fall on the open trapdoors I have at the entrance but they only do that if they are almost on top of me.
The way it is now, it forces players to wait hours for mobs to come and die so you could have enough mob essence to build the stuff. I feel the 7-8 hours I played yesterday, 90% of the time was waiting for mobs and slowly slabing every cave near my 'base' so I could increase the mob spawn rate around my traps...
Or I could just be missing the obvious way of doing it, for which I would be very happy if someone throw me a few tips!