Raemara's Comments

If you have a character that hasn't completed Uldum, I recommend taking that character because I had one that had completed Uldum and had phasing issues. Had another character that had only done 46 of the quests and they could engage the ones that were phased out on my other character.

I think Blizzard is trying to be funny here because if you think about what it says about the grub, it says they are little green grubs hence possibly why there are no rares.  Just a thought.  I guess Blizzard wasn't trying to be funny, they just had an unfixed bug :)

Take a character that hasn't been to Twilight Highlands to the Devouring Breach, very few mobs and lots of Twilight Fiendlings visible.  No phasing issues.