Not sure how to reproduce, unfortunately, but it seems to be the same issue as MC-160978. That report doesn't have steps to reproduce so can't confirm it's the same issue. In short, one fox has food in its mouth but never eats it; other foxes in the same area do not have this problem. Using /replaceitem to force a different item into the fox's mouth works, and it drops it to preferentially pick up food, but still never swallows it.

Can upload the world if required?