Two things can be happening. In the console when u hit the ` key you can type AI on and that can fix it or sometimes the pathing messes up and the way I fix that is save the game exit ti the load menu and log back in. That restarts them and they aggro again but Id restart away from the mobs that can be kinda bad if you log in and all of them come at you before u have a chance to defend yourself

Yep, sometimes the AI just craps out. I have encountered it in both the vanilla game and in both RE scenarios, so it must be a glitch in the game itself.

In my experience. using the console to restart the AI doesn't work often, but exiting to the main menu and reloading the save always does. So it's a bit annoying, but it's easily fixed.