When setting the max health of a player on PlayerDeathEvent or on EntityDamageEvent (when the damage would kill the player) and the player respawns afterwards it will show 20 max health for a brief moment before changing to the actual number that you set it to.
Example given below:

In the example I start with 54 max health (27 hearts) and reduce it by 6 (3 hearts) on death. On respawn the health is shown as 20 (10 hearts) for a brief moment which it then seems to update to the actual 48 (24 hearts) afterwards. Setting the health is done in sync.
 
I reduce the health by 6 with the following code:

AttributeInstance attribute = player.getAttribute(Attribute.GENERIC_MAX_HEALTH);
if (attribute != null) {
    attribute.setBaseValue(attribute.getBaseValue() - 6);
}