Highend theme ignoring header settings

We recently upgraded to 3.7.4 and now the header is appearing centered, even though the settings in the highend theme options are set to left.

Looking at the page source, it seems a flex box is being assigned to main-nav now, and the css properties are centering the content.

The site is https://www.essl.co.uk/ and I’ve applied some custom css to align it to the left, but this seems to be a bug that needs investigating.