this happens what seems like at least 33% of the time. I get the notification that such and such settlement is under attack, I fast travel there, fight off the raiders/synths, etc., talk to a settler and they tell me thanks for helping them, then I fast travel out of there and it tells me I failed the mission. After a while I started running around outside the perimeter and finding some random stupid mutant hound sitting around 1000 meters outside of a settlement and that would finally check off the defend settlement task successfully. Many other times, though, I run well outside of my settlement perimeters in 360 degrees around them, LB clicking furiously in all directions, to no avail.

I wouldn't bring this up but the fact that this happens sooooo often and at least a dozen times now I can't even find the last raiding straggler even when I walk halfway across the commonwealth trying to find them.