What @Fitzik mentions does happen. I have seen the hearthlings pick up raw food instead of cooked food many times, when there was plenty of cooked food available. The only reason it seems, the raw food was closer.
Althought I always considered it good. It gave variety to the diet.
Have fun, Kyth.
Update: Just to check if the wrong eating behaviour was due to something else I unistalled the game and installed it again. It now works as it should. Hearthlings eat cooked food without problems.
Have Fun, Kyth.