The bug

The check to see if the player's hearts should shake only takes into account the actual health bar, not any health granted by the absorption status effect.

Steps to Reproduce by Darth Cobby

1. Ensure you are in survival or adventure mode:

   /gamemode survival

2. Disable natural regeneration:

   /gamerule naturalRegeneration false

3. Damage yourself by running the following command three times:

   /effect give @s instant_damage

   → Your health bar begins to shake

4. Give yourself the absorption status effect:

   /effect give @s absorption

   → Your health bar continues to shake, despite your total health being greater than 4

Code analysis

Code analysis by bemoty can be found in this comment.