Yes thats the mechanic for them, but OP isn't talking about that. He's talking about when the mechanic doesn't work, which is a big glitch in the game, I've seen it many times where they physically won't stun from overload rounds, even divinity. Same goes for barrier Champs, I did a solo lost sector the other day and had a barrier Champ continue popping his bubble every 2 seconds and wasn't stunning (was forced to perfectly time a thunder crash). It's usually not too big a deal for good players, as we can adapt, but lower skilled or powered players might not be able to counter it

That's not the issue. If they are stunned, they should not be regenerating health. In many cases they continue to heal even when stunned.

In my video above (top post), I get the message that I stunned it over and over and over but it's still regenerating health.