I've actually noticed the same. I am coming from Chrome as it is my favorite browser. Vivaldi is so amazing, I actually like it better than Chrome in some instances but one of the annoying features I've found is when doing a task on chrome, sometimes I'd get a small window and it was convenient. When doing the same task in Vivaldi, I get another tab opened, the focus is switched to that tab and it covers my whole screen taking my full attention off the main window. Sometimes I just need that small window and if I want it as a tab, I'll drag it to the top with the rest of the tabs and make it a tab.
As far as a fix or a setting to turn on/off this feature, I hope something comes along. Another thing I have gotten into the habit that makes my job easier when working is have two windows of the same browser open side by side. I could click and drag a tab off the top, disconnecting it from my main window to create a new window of that site. Not the case in Vivaldi.