@robertgr: I have now discovered that if I launch Vivaldi from a hyperlink then every other time I click a hyperlink thereafter I get the correct behaviour, i.e. the new tab takes focus. More significantly I can re-enable the extensions at this point without issue. If I launch the Vivaldi from a shortcut then I see the issue (further testing has shown that the even with extensions disabled after an undetermined amount of time the issue recurs.
Follow the big releases and minor updates to the Stable version of Vivaldi for desktop.
You'll find the blog posts also on Vivaldi News - Desktop Releases and Vivaldi News - Desktop Updates.