Are you using an ad blocker in your browser? Your ad blocker is blocking the ad but not the overlay
Opera using the privacy protection tab built into the browser, but with Safari on iphone had no idea you could run an ad blocker.(not fussed about phones in general and don't use apps)
I try and run ads to support the site on my iphone(if I'm not browsing for long periods on the site) but it just overwhelms the resources of my phone, becoming sluggish and feels a little intrusive.
My work around as mentioned before is disabling JavaScript and the site runs really well and snappy without ads---maybe this is causing it on the iphone so will try again with JavaScript enabled.(Yes it is an old iphone 6 up to date up until yesterday when a new 247MB update has just arrived, yet to install 12.5.3)
Not really fussed as it only occurs the first time i click past the intro page, for example----I click on the first alert when I log in and it happens without fail, refresh the page and for the rest of my visit everything is fine.
Disabling JavaScript allows me to also read news articles online without limits on my iphone.