I'll note right at the start that this bug report contains SPOILERS ABOUT AN ENDING. Like literally, the dialogue in question tells the whole ending.

You've been warned ------------------

Okay, So I followed the Brotherhood of Steel storyline to the T, without any variation. Anything they asked, I did it. Even killing my mentor, props to Todd and his team for making that very emotionally tolling. After the final mission assaulting the Institute and the explosion of the reactor, I've returned to Santurary to finish up the Minutemen missions.

However I have a problem. Anytime I walk up to Preston he'll begin a dialogue talking about how he's surprised the Institute is gone and judges for killing even the civilians and blowing the place up. This dialogue will come back up regardless of how many reloads I do or time passage.

This is a problem because I wanted to bring him with me to do missions, I'm no longer to start any dialogue other than the endgame conversation of the Institute.

Although, if I go and complete a mission to unlock a settlement and come back to report to him, he'll start the dialogue to give me the next mission. Wait a second, and then tell me his feelings about the Institute again.

Having worked with Creation Kit on Skyrim, I'm assuming his dialogue check to see if he's said this before isn't working?