To the OP - please tell us what kind of trees were the stumps from?
Chickens eating tender moss is a given, but there are varying thicknesses and hardness of bark depending on the species of tree. There are thin, tender barks such as aspen and willow and very thick, woody barks such as ponderosa pine and redwood and cedar. In dead and downed trees, it's a given that bugs will be lurking underneath bark, and chickens will enthusiastically peck away at the bark to liberate them.
They may eat bits of thinner bark, but it would be rare for chickens to make a meal out of it. If they do, it shouldn't harm them any more than when they eat wood shavings that they find in their coop as bedding. As long as chickens have plenty of fresh water, they shouldn't have any crop problems from it.