Same, here @natanvarga
I noticed it depends on the incognito mode (private window): each and every blank tab is normally filled with its' correct thumbnail, once that same page is opened into a normal (not private) window.
That is: if you normally use Vivaldi in incognito mode, all the TAB Thumbnails (and speed dial thumbnails, as well), will remain blank, but as soon as you open those pages into a new normal (not private) window, the thumbnails will appear into the private window, as well.
It seems that private mode doesn't render the thumbnails
newscpq
P.S:
I'm using Vivaldi 64, on W10.