Yes, i too have met those NPC's in the single player campaign.

Proof:


I met about 4-6 such NPCs throughout of my campaign (maybe more, i don't remember exactly).

Though, i don't think this is some kind of bug. And even if it is a bug, it's a quite good one. Since this renders one or two of three elements useless against NPC, thus making you to use different tactics or weapons to beat them.

In matter of fact, i don't rely only draining NPC's HP. I also rely draining their energy (if they happen to use energy shield) but mostly, i rely on overheating them and skipping their turn completely. (It's great if you are enable to render your opponent unable to fight back.)

NPCs in range of 40-49 aren't as challenging as NPCs above lvl60, especially Insane difficulty mechs. From that point, the real fun (frustation) begins.

--
Aeacus

It's as Aeacus said, the true insane ones are the ones beyond lv 60.
And the reason of this is the NPC's don't have any limitations about resistance modules, so they can equip a lot of them by luck. If you have noticed, they have less energy and heat stats when they have that big of a resistance. This is a big drawback since you can easily turn their shield off when you fight against them, and get some advantage by depleting their energy or overheating them.