Expected:

Armor stands (like all other living entities) should check if they are invulnerable first before checking anything else when damaged.

Observed:

Armor stands (unlike any other living entity) requests NonLivingEntityDamageEvent handle and checks if it is canceled before anything else.

Steps to reproduce:

1. Spawn an armor stand using either org.bukkit.World#spawn method, org.bukkit.World#spawnEntity, or spawning manually through NMS classes.
2. Set either or both of the invulnerable and marker properties to true.
3. Damage the armor stand in any way.

Build number:

Paper version git-Paper-175 (MC: 1.14.4) (Implementing API version 1.14.4-R0.1-SNAPSHOT)

Notes:

The issue is at net.minecraft.server.v_?.EntityArmorStand#damageEntity.
Take a look at the differences between this method and net.minecraft.server.v_?.EntityLiving#damageEntity.
Pretty sure whoever coded this was trolling since it doesn't match any of the other damageEntity methods from other classes extending EntityLiving.
(It may also be worth checking net.minecraft.server.v1_14_R1.EntityLiving#damageEntity0 and
net.minecraft.server.v1_14_R1.EntityArmorStand#e, the latter of these is missing its de-obfuscated name as well)