Previous post button display issue

Hi,

I changed to side navigation header and I now have an issue with the ‘previous’ button for Posts and Portfolio items when viewed in smaller screens (see attached image). Laptop size and iPhone size is fine, but if I reduce the width of my laptop browser, then that happens. The button appears in the middle of the screen instead of the left. Any idea of how to solve it? I tried removing all custom CSS to see if any of my codes was causing the issue, but still happened.

Example blog post: https://fefobouvier.com/apod/

Thanks,