marbrueck

This is the world in which I reproduced the behavior. The data pack is also present and just needs to be activated.

Here is an excerpt from my datapack code, which adjusts the CustomName every tick, based on their health.

execute if score @s mh.mobhealth matches 1 run data merge entity @s {CustomName:[{"atlas":"minecraft:gui","sprite":"hud/heart/half"}]}
execute if score @s mh.mobhealth matches 2 run data merge entity @s {CustomName:[{"atlas":"minecraft:gui","sprite":"hud/heart/full"}]}
execute if score @s mh.mobhealth matches 3 run data merge entity @s {CustomName:[{"atlas":"minecraft:gui","sprite":"hud/heart/full"},{"atlas":"minecraft:gui","sprite":"hud/heart/half"}]}