It seems the game does not consider lettuce plant matter. Completely edible by carnivores.
Why does it seem like that to you?
Its not red for a carnivore and eating it causes none of the penalties every single other piece of non animal matter does. Ate a bunch of it in my game to make sure. Was a very strange experience.
It’s appreciated if you add the version/build number to a bug report since stuff like that can change quickly.
I can confirm this bug for 0.E-6779-gc3caa74 (build 11094), no mods.
… And I have not the slightest idea what’s causing it 
… The material is “veggy” and all its entries are also found on other items that are considered inedible with the carnivore trait.
Edit:
Actually, it’s the low calorie value… Since it checks if the nutrition is higher than 0, it will not recognize it as inedible.
As for why it considers 6 lower or equal to 0 - no idea…
I think calories are modified for carnivores, so they may become 0.
It shows as 4 kilocalories in game, the same for normal and carnivore characters, and eating one as a carnivore does indeed add 4 kcal to the stomach and (later) to the stored body fat…
Still not sure what happens there…