I'm playing the Taking Independence quest given by Preston (the one where you have to kill the mirelurk queen).
What happened was that I fast travelled somewhere else just before facing the queen cause I wanted to get my missle launcher at Sanctuary. Right after that I went to the Gorski Cabin on accident then all of a sudden, I see Preston following me around when he's supposed to be at The Castle.
Reason why I'm surprised is because I currently have Curie as my companion. Preston can't be commanded or interacted with (gives the "We have to focus on taking the castle" line) but he still shoots enemies around.
I think the reason why is that Preston counts as a companion at that moment and he still works as a normal companion (but can't be interacted with).
I don't know if this is a thing, or it's just in my game