Natural regeneration heals one heart instead of half of heart after taking at least a heart of damage. By looking at the first video below, I first fell 5 blocks to take 1 heart of damage, and natural regeneration healed 1 heart. Then, I fell 5 blocks again and I took 1.5 hearts of damage, and natural regeneration was healing half a heart a bit faster than normal regeneration. Next, I fell 17 blocks and natural regeneration healed 1 heart, and then I used instant health potions. The potion healed 3.5 hearts instead of 4 hearts due to natural regeneration healing incorrectly. When the instant health potion is used, natural regeneration heals one heart again. In the second video, without using potions, I fell 22 blocks to take almost all my health, and the natural regeneration heals 1 heart, but then the natural regeneration proceeds to heal the correct amount of health after that.
Without the video, I tested this on 1.16.0.64 beta on my phone and natural regeneration healed incorrect amount of health again.

Video 1: https://1drv.ms/v/s!AjwyWqM1S9ODmQfTcZy2akuUhgrV

Video 2: https://1drv.ms/v/s!AjwyWqM1S9ODmQiO7MB6zOcrz4Ju

Steps to Reproduce:

1. Go to any world or create one.

2. Take at least 1 heart of damage.

Observed Results:
Natural regeneration heals 1 heart.

Expected Results:
Natural regeneration should heal half a heart at a time.