Sections Disappearing (Caused by Header)

As you scroll through various pages of the site, sections of elements start disappearing (the sections that disappear appear to be the same size as the header).

It happens when you scroll down the page then back up or reload the page when you are lower on the page. It seems that wherever the header is on the page becomes the section that disappears. I’m not sure if this is a problem with the theme or something else.

I already reached out to our hosting company that says everything is fine on their end.

Thanks,