Header resizing itself on listings.

I am having an issue with the Download Theme not correctly displaying the header on a site I am using it for, not sure why it just started doing this recently.

On all pages the header appears as intended, yet when a listing is open, the product page where users make the purchase, the headers banner image shrinks to a fraction of its size for some reason. as shown in the attached image. It is just the listing pages that are affected, nothing else.

Disabled all plugins issue remains.

Searched the forums already and not found anything on this.
If anyone can help it would be appreciated, thanks.