Bug or something else? (Fallout 3)

I have a license in the incentive. The story is almost over, but in the purifier, instead of going to Fox to launch the purifier, because he is immune to radiation, there is Guardian Lyons instead. And if you go there yourself, then after the credits the end of the game is thrown into the menu, although Broken Steel is installed. I reinstalled the game, and started all over again, switched the dls off and on, everything was useless. The video is attached at the link
Spoiler
There are a couple of mods. I don't think they have any influence, although the devil only knows. But all of a sudden someone came across this, how they corrected it, what they did, and the like. I will be grateful :)