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