Header parallax image jumps when page first opened.

I have a large parallax image at the top of a page - please see:

https://advancedhyd.com/about-us/

When the page is first opened the image jumps - it moves down to show the top part of the image and then moves up again.

I have a custom height set (600px) for the top section. The actual image is 1920px x 1085px.

How do I stop the image jumping?