Welcome to the forums @JayJayNinetyFive
While not an official answer, https://luhn-website.netlify.app/ could appear “green” or “not green” depending on the time the site is checked. I initially received a “green”, followed by a “grey” rating. This is because the the site is served from a global CDN, some of which are green, some of which aren’t. The same would apply for the custom domain you use too.
I believe you will find the following thread of interest