I don’t know if it is a bug or not yet, but I have noticed that sometimes when I work with multiple viewports in the Model Space, when I switch in between the viewports, some of them zoom to the extents automatically. This kind of behavior did not bother me too much, but I got a drawing that presents the issue on all the viewports.
After a little bit of investigation I found the culprit in a variable called UCSFOLLOW.
By default the variable is set to 0, but in the current version of Civil3D when working with multiple viewports I found it more than often to suddenly change its value to 1. It is important to know that this is not a general variable or an overall dwg variable, but it is a viewport based variable, so that means that it can be enabled in one of your model space viewports and disabled in the other ones.
Since I cannot pin point what causes this variable to change its value yet, I just thought to make you aware of its presence.