We used to share selected collections to customers to allow them test some features. But now share with read only access is suddenly disabled for Basic plan. Allowing me to add only editors, which then have suddenly access to whole team projects, not only collection I clicked Share.
Hey @coding-bear 
Welcome to the Postman Community! 
Were you previously part of a trial or some other plan type?
For the Basic plan type, that doesn’t include Role Based Access Control so from what I can see in the image, it’s working as expected. 