Normally (in my setup) it shows a list of "most visited" urls.
The new behaviour is to still show this list but omit any amazon.co.uk (and maybe other, I have no idea) urls.
Completion works as expected as far as I know.
My "new tab page" is always set to just blank page and has been for years. I dislike all the fancy new page stuff and just set it to blank page on initial install.
The sites that show up in the urlbar dropdown are frequently visited ones, same as the ones that show up when I turn the "update1" stuff off, or the ones prior to ff 75, just not sites containing amazon.co.uk.
So while I don't doubt your information, it doesn't make sense to me as an explanation. Maybe not having any sites in a new tab page is some kind of corner case that exposes a bug. It's weird that amazon is specifically affected though!
I have a browser profile that is used almost exclusively for amazon content as well. The most visited links in that profile are amazon.co.uk and prime video but the only "most frequent" links it would show were on twitch and the occasional youtube page I visited to find trailers for amazon hosted videos.
When I went to about:config and toggled the four "update1" bools to false, amazon came back in my "most frequent" links.
Pretty strange, eh?
Edit: so I just installed 76, and switched the four "browser.urlbar.update1" bools that enable this back to "true", and again, any amazon.co.uk links are removed from the list I get when clicking on the empty urlbar.
I don't really care about the ugly styling, but since this is a navigation feature I use extensively, I'm turning the whole thing off again. Hope they fix (or document) this before the choice disappears!