Issue
Using a datapack to add max_health as an apotheosis tier augment correctly applies that entity attribute. The entity that spawns is not, however, actually set to that max_health.

Exampe
The attached datapack snippet for the ascent tier causes zombies, skeletons, etc. to spawn at 20/60 health.

{
"type": "apotheosis:attribute",
"modifier": {
"attribute": "minecraft:generic.max_health",
"operation": "add_multiplied_total",
"value": 2
},
"modifier_id": "apotheosis:ascent/max_health",
"sort_index": 100,
"target": "monsters",
"tier": "ascent"
}

Proposed solution
Following entity spawning, set the entity's health value to entity's getMaxHealth.