If you reload the page the issue does go away... but only technically.

The issue is the page references a few images over http, which redirect to their https form with a permo redirect so the browser knows the new location when you refresh. Your browser also now has the image cached too so doesn't need to request it

One of those two seem to make it stop flagging the problem. I guess the browser only complains if it needs to make the request

@OP: I'd do a search and replace http://domain to https://domain and try to use relative urls where you can in future (I am aware this was probably a WordPress thing, not a you thing!) - My go-to for this is https://wordpress.org/plugins/better-search-replace/ for the content side and a good ol text editor in your theme files if needed, should be good to go :)

Thanks, I'll definitely look into that.