After reducing the attribute generic.max_health of a player, the player's HP will not be capped automatically until taking damage. If the value of generic.max_health is lower than half of the original HP, "client-side fake death" will happen.

Steps to reproduce:

1. Reset the player's state

/attribute @s minecraft:generic.max_health base set 20
/effect give @s minecraft:instant_health 1 3

2. Modify the attribute

/attribute @s minecraft:generic.max_health base set 6

3. Check current HP

/data get entity @s Health

    The result is 20, not 6
4. Take any damage
    For example, step onto a magma block. After taking damage, the player's HP will be 5 and the "You Died" UI will pop up.