Steps to reproduce:
1) Put a shield in offhand and a food in your hotbar, and get to a state where it can be eaten (either with can_always_eat or hunger)

2) Constantly guard with the shield and never stop. Now swap to the food in your hotbar without stopping blocking

3) Let the mob break the shield, and still do not let go of right click. You will begin eating the food as expected, but once the eat duration ends, it will just... continue chewing

Note: The client in the video is modded, but the bug has been confirmed in vanilla.