OK. So we got there in the end. The process they document to get an unlisted app listed is not right. We had to make a copy of our existing app that is distributed via ABM and change is icon and package name.
The correct steps to follow are
Submit your app for review (pending developer release). Put in the notes that it is to be an unlisted app
Submit the unlisted app request by filling out the form
Get the app passing review for everything but rejections quoting guideline 3.2 (Our app actually got full approval without the unlisted app status which it shouldn't)
If you get the app past review do not deploy until you get an email saying it has been approved as an unlisted app.
5) Release your app
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect
Tags: