In the following particular instances, it is not a bug:
In Nightmare Mode, Huge Grenade does in fact deal weapon damage (rather than the spell damage it deals in Story and Hard Modes). This was an intentional change for Nightmare Mode, done to make the encounter more challenging.
As for Terminate, it has always been a weapon damage attack in any difficulty mode that it appears, and it would have never worked to Force Shroud it - any such luck to the contrary is a mere coincidence (though probably not uncommon).
In other instances while Force Shroud is active and you know that you are being hit by a tech or Force attack, the issue is most likely that players and NPCs always have at least a 5% chance to hit with any ability, regardless of how high your defense is to that type of ability. So while Force Shroud is active, there is always at least a 5% chance that you could be hit by any tech or Force attack.
I don't like this anymore than you do. I know it's extremely frustrating to be hit by an ability that, for all intents and purposes, should not have hit you. Unfortunately, fixing this issue is not as simple as it seems, but I'll take a deeper look to see if there is anything we can do about it.
So basically the ability is not infallible no matter what it says? and any ability force or tech can hit no matter what accuracy they have. That's a bit ridiculous to be honest.