I recently updated all of the NuGet packages in one of my solutions, and it seemed to break the CSS.
I recreated the project and copied the model, controller and views across to the new project. I then updated the NuGet packages one by one, and when I updated to Bootstrap 4.0.0 (or later) it broke!
When I downgraded to 3.3.7 it all came back again 🙂
Therefore, it is probably outdated code in the MVC template in Visual Studio that causes the new bootstrap to break it.
Hopefully this will help other people having the same issue