After something hurt you, health stops regenerating and stays at 9 1/2 

Steps to play it: apparently it is only in hard/hardcore 

To do it faster, let a mob hit you 

Regenerate your life and you will see in the HUD that not all life is filled 

Something worth highlighting:1 in the video (Minecraft bug) you can clearly see that I lost all my hearts but for some reason, I didn't die. 

2 the screenshot is in a world (the first one I died in) in that world I was never attacked by a drowned, and even then my life did not regenerate to the maximum after a while. (I deleted it after I died xd)