These are called Favicons. I am also having issues with Favicons.
My Favicons are blank in my Lastpass Vault and also my Firefox Lastpass Websites. I have changed my Master Password to see if this would correct, it didn't.
I am also having an issue with Recently Used web sites. I clear my Recently Used web sites, close my Firefox Browser (55.2.2 (64-bit), relaunch my browser and my Used Sites have re-propagated.
I am also having an issue with having to Login every time I launch my browser. I have checked Remember Email and Remember Password. This is not working, as I stated I have to fill in my email address and Master Password every time I open my browser.
Just to confirm this is very not just limited to the site above - added another site today and again it shows the icon perfectly in the pop-up when saving the password but then doesn't show up in the vault.