Hello @dcasalotti
I really hope that this is something that gets addressed soon. I am very sorry you are running into issues with this also.
In my testing, I have found that the best way to ensure your question banks from Classic Quizzes migrate to Item Banks for New Quizzes properly is:
1 ) Create a classic quiz. You can use your sandbox for this if you have one.
2 ) add all of your questions from a question bank to the quiz.
3) Save the quiz. You don't need to publish it as this is just more of a quiz shell we're going to use to house the questions from the bank.
4) Then export it from the course. This will export the entire quiz and all the questions contained within it as a QTI file
5) From there you can import that same QTI file into your Item banks you will use for the New Quiz engine.
It's quite work around! But I have found that this is the best way to ensure my quiz questions I was using in the Classic engine get brought over for the New Engine for future use.
-Colton