Comment on Technical Support by SEO Dave.
Upgrade from Stallion Responsive 8.2.1 to Stallion Responsive 8.4 and the issue will be resolved. You should see the update available under your WordPress Dashboard.
If you use a caching plugin don’t forget to clear your cache. I got the “Large ads above the fold on mobile” warning a couple of weeks back on a site that’s not very popular, the cache of some deep content hadn’t refreshed since I upgraded.
The issue was Stallion Responsive was set to load the large 300 x 250 AdSense ads on some mobile device sizes. The fix was a few minor CSS tweaks to the mobile.css file so smaller ads are loaded. Switched them to the mobile specific AdSense ad sizes.