I don't know exactly what happened here but clicking the plus button to add a new tab, it will open a new window. It used to open a new tab but now it doesn't. Happens only with file explorer.

I did however update my Windows to build 17618, so I can try Microsofts version of tabs. I disabled Groupy but Microsofts tabs was not showing up, no setting for it either. I edited a registry file to activate the tabs feature. Restarted the computer and tried it out. Didn't like it much at all. Glitchy/slow etc etc. I turned tabs off and went back to Groupy and that when I noticed the issue. Is it Windows or Groupy that is causing it. I tired checking Groupy settings but nothing is out of the ordinary.