What I expected to happen was...:
Elder Guardians from spawn eggs / command should be spawned with full health (80 health, or 40 hearts).
What actually happened was...:
Elder Guardians, although they are spawned with a maximum health of 80, they only have 30 health, the same amount as a normal guardian.
Steps to Reproduce:
1. Go in a creative mode world and use an Elder Guardian spawn egg to spawn an Elder Guardian
2. If you kill it with a sword you will find it dies in a lot less hits than you'd be expecting because it is spawned with less health.
3. You can check this with the command:
Screenshot is attached. Near the top you will see that it's attribute "generic.maxHealth" is 80, which means its max health is normal. However, about six lines from the bottom, you will see "Health:30.0f", meaning it is spawned on less health than it's max.