Sorry the site is only local at the moment.
However I can see the potential reason. In customise layout /header I have “mobile header” set to “off” That is because the mega menu has the settings for how the menu will look at mobile size and setting “mobile header” to “on” adds the logo and search icon and that wasn’t what I wanted.
Anyway with mobile header set to “off” it is working fine and also works OK with GP 3.0 with floats. But when I turn flexbox on then the mega menu is hidden on mobile. I suspect that is the more correct behavior given that “mobile header” is set to “off”
I have now set “mobile header” to “on” with flexbox enabled and my mega menu is now there but has the logo added, a search icon and an extra menu toggle button. I can hide all those with css, but it was a lot neater with the mobile header “off”.