Have a look at https://comptonshawford-pc.gov.uk/responsive-table-test/ on a large screen.
As you resize the window,
first the sidebar is relocated below the page content (correctly)
then the text in the table cells suddenly stops flowing into several lines.
So the table cells become very wide, which makes them difficult to read (especially on phones).
I've tested this with Safari, Chrome, Firefox and Opera on the Mac and Safari on the iPhone.
Other themes that I've tested work in a much more predictable and user-friendly way (Twenty-Fifteen, Twenty-Sixteen, Responsive, GovPress)
I had a quick look at the code and couldn't figure out how what the problem was
In case I am testing a different theme when you visit the URL above, here are two screen shots
With Franz Josef