So, I tested by adding a panel menu to the second row. That didn't seem to fit either. So, I added another flex grid and left it at the default. Added a responsive menu and that seemed to go in OK. When I preview it, it looks fine - it's just the WWB layout that doesn't seem to be very WYSIWYG
I ended up with this:

So, what is going on? I am sure it will be something simple, but I can't see it.
Demo project can be found here: https://www.dropbox.com/s/hie7acxlcgzj5 ... 1.wbs?dl=1
Oh, and I'm using the latest 17.1.2 version.
cheers
Alan