I have some invulnerable mobs spawn inside my cursed earth grinder and they just won't die..
they also don't appear to be despawning.. there was a zombie and a skelly and they didn't have health bars over.. (regular vanilla ones)
I could hit them forever without hurting them, also grinder can't kill them and gets stuck on trying to kill them which never happens..

ok so I found out what this is about, it's a know bug fixed in next version of MFR.. here's a screeny that explains it..

http://postimg.org/image/jvu9fcojh/