It is a simple matter of breakpoints. The TopBar menu items are wrapping onto a second line (which you cannot see as they are the same colour as the white background) at approximately 1000px.
As @Oscar says above - you should not need to use the built in Stacks responsive settings at all. The Foundation breakpoints are sufficient.
I also notice that you have a 30px white border around your 1 column stack. You should not need to do this, just use the row padding settings in the stack and the white page background will show through. This will be much easier to handle responsively as I a 30px border on a small screen will take up a lot of the screen width.
Below is a link to a video showing you what is happening. I have made your page background a grey colour so that you can see the menu items wrapping to the second line as they run out of space. I hope this enables you to visualise what is happening a bit more easily.
Top Bar resizing video: https://infinit.io/_/3mxcDfz