Mushroom stew doesn't stack if you milk a Mooshroom (WAILA reports that it's part of MineCraft) , but when you make Mushroom stew with mushrooms it gives you stackable bowls (up to 64) (WAILA reports that it's Natura's so I am guessing it's your bug).

Just tested with Mushroom stew in a tank, also gives me non stackable Minecraft stew.

Not sure if this is a feature, nerf, balancing or something else to be honest.

I thinks it's just an oversight. @progwml6 ?