So I've had to reload the game three times now, but no matter how many times I've tried Defend Sanctuary Hills will not complete. A group of Synth has attacked it, I show up and kill them along with a Settler that was apparently a Synth. After I kill the enemies I talk to one of the Settlers in town, they say thanks and good job. When I walk by a few other settlers they talk about how we beat the Synth. Quest still does not complete, and if I leave the area it says quest failed and everyone in Sanctuary Hills suddenly dies?!

Hope this gets fixed soon, I currently cannot progress in my game at all.

Same thing was happening to me. After it popped up saying it failed I went back and everyone was still there even though the map said there we're only 4 people left. In workshop mode all stats were about the same. Just had to repair my turrets.