How to override theme for quicktabs which actually renders a view
As Sunil says you are probably using view variables in your block template

Perhaps look for another template to override further down in order to find a views template to override and use your html/twig code



Here is what I would do. 

Find the first template inside views-view.html.twig in my case it is views-view-unformatted.html.twig (red arrow above) and copy it in your theme templates folder.  Clear cache and make sure it is used and than explore the possibility of renaming it to be specific and used only for the quicktabs block.

See here for more info on how to name your templates