I had a question from an instructor today about this issue, and could not figure out HOW they had even made a New Quiz open in a new tab. Turns out they made an assignment, selected "External tool" from assignment type, and selected an option called "Quizzes 2." This was the original way of making a Canvas New Quiz.
We have now deleted this option from the list of External Tools. However, there does not seem to be a fix if the person selected the assignment to open in a tab before they selected the Quizzes 2 tool. I will be following up with support@instructure.com to see if they have any ideas.