I too have had them die, but I suspect what is happening is that they are getting damaged by a hostile NPC, and then friendly fire from a turret kills them. Protected means: they can go to nearly zero health but not any lower, EXCEPT by damage from the player. Seems "turrets" count as player
As for the "Help! We're beign attacked by two bloat flies! Come save us Sole Survivor!" settlement defense issues . . .
I'm as annoyed by it as anyone else, but not knowing how much of an effort it was to bring this into their development in the first place I won't place too much criticism for its deficiencies. It is a pretty big step I think and it is effectively transforming this franchise from a character-driven largely static world game to one which has the elements of a builder/strategy or tactical game.
Hopefully they will remediate the deficiencies in a DLC, but even if they don't the mods that this opens up are bound to be astoundingly good. With mods now projected to be available (eventually) for both consoles that is good news for all of us!
I am not sure what I have done with my settlements but I have not had a defense quest for a long time. I _have_ had little "mini-attacks" that ended almost immediately several times while I was visiting a settlement but no popup "Settlment is under attack" for a long time.
I suppose it might be that I have ~150 defense ratings at most of them? Or it might be that I no longer live in Sanctuary and rarely interact with Preston or the Abernathys or Tenpines (where most of the "rescue, etc." quests seem to come from?).
For a while I thought it was the fact that I was keeping my settlement footprints really small, like literally turrets, shops, buildings, stairs, crops for 20 settlers all packed inside a palisade so densely it could almost not get any more packed in. But I've subsequently expanded the palisade on several of them and no obvious change in the frequency of attacks.
In other play throughs I was getting spammed with them alot, but not this one.