instead of this:
This happened on a fresh install of the latest Vivaldi on macOS Tahoe.After searching the web and following a few suggestions with global website permissions, preferences file, etc. - which all didn't work - I finally landed on a post that hinted to a shortcuts setting:
In the Vivaldi of the 1st screenshot, it was unchecked.Checking this setting made the CMD-keys return in the menus.

A new version of