It is very simple to duplicate. Go to the URL you gave me, set a big radius like 8 to be sure you can see the problem then use the example index.html that comes with it and notice everything has a radius. Attempt to create a button without a radius and there seems to be no way. Either everything has a radius or nothing has a radius.
I looked further into this. It seems it has been this way on all versions of foundation 6. I have been working with foundation 5 for so many years and I am just now starting to see the negatives of foundation 6, or possibly just my lack of knowledge. Is there a way to specify radius or not on a button? A callout? A text input? Can I have two buittons next to each other with one having a radius and the other not having a radius?
Back in foundation 5 we use to use class=“button radius” for a button with a radius or class=“button” for one without a radius. Now radius means nothing. Use it or don’t and get the same result.