Applying any level of health boost to a player or mob resets their health to their original maximum health. For example, if a player originally has 3 hearts of health and is given health boost, he/she will be healed to 10 hearts in addition to gaining the extra hearts. Health boost is an effect for increasing maximum health and shouldn’t have a healing effect.

Steps to reproduce:

1. Take damage with any means. I recommend using /damage @s 19 void, which should reduce you to half a heart of health.

2. Apply health boost to yourself: /effect @s health_boost

Observed results:
The player is healed back to 10 hearts of health.

Expected results:
The player should keep their current amount of health.