of course they do. They have to be forbidden to go into the kitchen taking those. If you are a prisoner sitting at a table and people telling you to only ♥♥♥♥-paste, but there are fine meals just on the able, of course they grab them.

Get the mod "LOCKS" and lock the kitches so they stop walking in there. Also reconstruct the paste dispenser and the fillers. But as long as they can reach better food, they going to.
They are pirates, crimimals, murders. They wont be stopped by a little sign telling them to not eat the food

Its not even a bug, its vanilla pathfinding/food finding behauvior. Prisoners are like "wild men" looking at food finding. They search the map for best available meals and eat them, like wild mens wandering trough you killbox to eat the simple meal somone dropped being drafted during a raid