Sometimes when you take damage from enemies or fall damage and your Hunger bar is full, your health bar will not regenerate, leaving your hearts low. The game won’t let you eat because your hunger bar is already full but your hearts won’t regenerate.
A workaround for the issue is to run around until your hunger bar drops a little and then eat something to get your health bar regenerating again.
This seems to be an issue of an imbalance between hunger and health where you can have a situation where you are caught between values that would trigger health regeneration or trigger hunger to drop allowing you to eat.