I also saw such an odd thing: I defended a settlement, was successful, and when I looked (in build mode) at a turret, it had 'Repair' highlighted. I then further looked around to see if something else was damaged, and then went to the turret to repair it - but it was repaired, and the repair option was grayed out. So something fishy is going on. Either the report is wrong, or the game damages things and at the end - if you successfully defended - resets the damage.
I helped defend a settlement (Three Pines, I think), was successful, and afterwards when I opened the build mode it actually listed 3 plants, that were damaged. I had to repair them. So sometimes things are broken, even if you defend.
And the 3rd case - Three Pines - I had overlooked the warning and when the fail message came up I could do nothing - loading an older save was not an option, too many things had happened: I had cleared the castle and was not so hot to repeat this later. The settlement was reported in the PipBoy with 0 settlers, 0 water and 0 food. I went to the settlement several days later to look at the damage - and when I arrived the 2 settlers were alive, and kicking well, the turret was active, the plants and the well were fully functional - and after opening build mode the numbers went to normal...
There are several bugs in the system.
Also - I think the attacks should not happen while the player is in a quest. This seems to work sometimes, but only when the player is in another cell. If you are on the surface somewhere, the attack will happen. I just was in a heated fight with several Supermutants at a satellite array during 'The Lost Patrol', when I had to abort the fight and fast travel to a settlement. Not ideal...
Attacks also happen, while you are at or near (near in this case means if the cell with the settlement is near enough to be fully loaded) the settlement. There is no warning at all and also no 'grace period'. The attack happens, and if you are lucky you will hear the gunshots and be able to run toward the fight and help. If not - you are in bad luck.
Also I question the balance in these things: I was relatively near Oberhausen (?) when I heard gunshots from this direction. I sprinted to the settlement - there were only 3 settlers, but nearly double the minimum defense with 2 defender stations and 2 turrets. I was level 14 at this time, and I play on 'Normal'. The attackers were 2 radscorpions and one 'Legendary Glowing Radscorpion'. Three radscorps with one being a boss, which stood for one minute near a heavy MG turret and svcked up the fire seems to be a bit much for level 14 and 2 settlers. Something glitched there, because after I won (using chems, just barely), they all were grateful because I helped them against the 'Raiders'??
All in all the system with all its numerous bugs and failures (cannot really distinguish settlers and see if they are assigned) should be fixed ASAP. It's a pity that a good idea - many people seem to like it - was implemented so sloppy and cheap...