I've had this happen with two different characters (Axton and Krieg) and I have no idea what's causing it. Essentially it's insane health regen (no Moxxi weapons, no health return skills), more than the Neogenator's natural ability can account for. I get health gated and without doing anything the red bar pretty much instantly refills.
You're complaining because it's working better than you think it should???
The Neogenator offers health regeneration that scales inversely with remaining shield capacity, but stops if the shield is depleted. Damage caused while the shield is active increases health regeneration. .
What's the purpose in owning a Grenade Launcher? The sheer inability to continue life without it ! ! !