I noticed on the collection page of my website that the side panel with filters on the left has a white element below the filters that overlays the entire website. It can only be seen when scrolling down past the filters section on the left when it overlays the logo (see screenshot for the white gradient overlaying my logo).
The issue here is your form and search facets have a higher z-index causing it to overlay the header. You need to lower the z-index on class .facets__actions. Look for class .facets__actions in your css file and change it to position: relative and set z-index: -1 you will see it is gone.