In the interest of science (>_>), I went to check out the Gobsquat Rat Meat Collection quest. Seems the Invisibility potion reward there too is bugged, just as with the Ruins Scavenger quest in the Ziggurat. Choosing the option removes your rat meat, but gives you no Invisibility potions.
http://img194.imageshack.us/i/ratmeatbug.jpg/
Worked fine for me. Both quests. I either take the "random loot", or invis, and it's worked fine. At least as of Wednesday, it did.