Natural Regeneration different when take drown damage

When you are on land having low health and the hunger bar is full. Your health naturally regenerates half a heart every 4 seconds. When you are drowning, run out of oxygen, and taking 1 heart damage per second, your naturally regenerates raise up at 1 heart every 4 seconds.

Steps to Reproduce:

1. Make yourself low health
2. Stay underwater until run out of oxygen and take damage
3. Keep your hunger bar max

Observed Results:

Players health naturally regenerates 1 heart every 4 second
Expected Results:

Players health naturally regenerates half a heart every 4 second