Strange scroll behaviour on gallery ligthbox

I am seeing some strange scroll behavior on my galleries. When I click on the gallery item to open up the lightbox the the ligthbox scrolls down rather than taking up the full screen. 

It does not seem to be in issue in Safari or on mobile but I have tested it and the behaviour is more or less consistent in Chrome, Firefox and Edge. 

Here is the link to the section with the galleries

https://www.sagacreative.co/services-1

I do have the following custom CSS installed to limit the gallery thumbnail display to one but I have removed that and it made no difference. 

Quote

#block-a86202234716652b86cb
{
.sqs-gallery-design-grid .slide:nth-child(n+2) {
    display: none;
}}

I thought it might be a smooth scrolling thing so I also removed the following smooth scroll code from the CSS but for some reason my site continues to do smooth scrolling 🤷

TIA

You can use this to Website Tools > Custom CSS

/* Fix lightbox scroll down */
.yui3-lightbox2 .yui3-lightbox2-content {
    top: 0px !important;
}