OK, So I found something interesting about the reavers rampage ability.
When fighting the demon commander two times yesterday as a reaver, I used my rampage ability on it, a couple times throughout the fight on the routine difficulty, on PS4. Each time i used my rampage ability and started to attack the demon commander with either the basic attack or dragon rage, I received no health.
All other enemies i've hit, I received health and the enemy has taken damaged. I thought he was invincible during his scream so I tried hitting while it was doing other things, still, no health.
This happened in both of the times I fought the demon commander yesterday.
One of two things I can come up with as to why this is happening.
1. The demon commander doesn't take any damage while a reaver is under the effects of rampage, thus couldn explain why no health is gained when a reaver under the effects of rampage receives no health.
2. The game doesn't registers the reaver attacks on the demon commander while under the effects of rampage and thus not receiving health, but still doing damage to the demon commander.
I usually look at my health bar to see if rampage is working since I first ran into it, and it seems that the rampage glitch only occurs to me when I use it on the demon commander. Because Im always looking at my health bar, i havent been keeping an eye on the demon commanders health while attacking him with rampage on.
I will try today and see if the demon commander in fact does take damage or not while under the effects of rampage if i can get another game that'll let me connect.
If the demon commander takes damage, then I can say #2 is a possibility, if the demon commander doesnt take damage, then it's number one that could be the issue.