Butcher Makes Invincible Slimes

I spawned a 10 slimes and when i used /butcher it said it destroyed the mobs, but the slimes werent dead.  So i used it a few more times and then the slimes became invisible!  When i hit them, they didnt turn red like they were hurt and i lit them on fire with lava and they didnt take any damage or die.  They just hopped around while on fire.
