What I did was compare my child theme CSS file mobile navigation code (near the bottom) with the mobile navigation code in the latest version of the parent theme CSS (I recently updated to the latest version of Magazine Basic).
It was not the same. So I copied and pasted the mobile navigation code from the parent theme CSS file to the right place in my child theme CSS file.
Don’t know if this is the proper way to go about it, but it worked for me.
Here is the code I now have for mobile navigation at the bottom of the CSS file: