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). Attached are pictures.

You've got the 'Combine dock icons with running applications' setting enabled.

This makes any running applications not appear on the RIGHT side of the dock if you already have a shortcut to them in the LEFT side of the dock. This ways there is no duplication of icons, and is just like the Windows taskbar does it.