Just failed a settlement defence. I got the warning that it was under attack so rushed over there and talked to the settlers. They spoke of gunners coming back to attack them so I had to repair and build defences, I built 3 new machine gun towers and it completed the "build defences 100%" part but then instantly failed the mission as soon as the gunners appeared.

Machine gun turrets killed gunners in seconds, no settlers were harmed but its still in my journal as failed.

in the defense quests, you're not meant to build settlements, but just to appear there and help them fighting enemies off.

your turrets etc aren't directly connected to these quests

Loaded previous save and ignored the damaged defences already in place and just stood waiting for the gunners. Still instant fail as soon as the shooting starts.

Reloaded and waited then killed them when they showed up, said completed this time.

never had one fail, but maybe that's when your last settler dies or stg?

i mean, i've been called lately to a yet raw, unbuilt and undefended nordhagen beach to find iirc 2 mutie enforcers, 3 mutie brutes and 2 normal muties, all with granades and machine guns, hitting on my (lvl 26 at the time) 2 folks and 1 kid, how long could they possibly survive this? short enough to look like an "instant fail" anyway

(mine, including myself, only did because i'm a filthy, filthy cheat