Overlay working on same pages, not others

Site URL: https://corn-apricot-zn88.squarespace.com/portfolio-test

I know overlay doesn't work the same on mobile, BUT it works on my people page and values pages, but not on the portfolio-test page!!! 

it is done slightly differently in the CSS - with the overlay in the portfolio-test being from the site styles of Squarespace.

Is there a way to get the portfolio-test to work like the others on mobile? 

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