For speed dials that don't actually have a thumbnail set, they now show a tiny favicon in the middle instead of a colored background with the domain name text in the middle. But, if there's no https://site.com/favicon.ico available on the site, the speed dial will show that blank page/document icon instead.
So, that's how things work now. However, you can still click the speed dial to goto its URL, click the heart icon on the address bar and choose a specific thumbnail for the page. (The old colored background with the domain name text in the middle will show as a preview in the heart menu. That is the "No thumbnail set" choice. You'll want to pick a different one in the thumbnail preview.)
Also, you can still download/copy an image, right-click the speed dial and choose to set a custom image if you don't like the blank document icon.
