When I use Vivaldi (not Chrome or Opera) the icon in the apps dock isn't copied to show it's running, like all the other icons for apps. Instead, there is a small white dot beneath the original icon in the dock. I've only noticed this on one previous application (out of about 30).

In the Content tab of the Dock Properties dialog for that dock, is the 'Combine dock icons with running applications' setting enabled or disabled?