When a Collection's Default Read policy is removed, the group is also deleted, which results in all policies pertaining to that group also being deleted. The end result is Items – which had inherited this Collection's Default Read policy – now have NO policies, not even for Anonymous.
1. Create Collection
2. Restrict Read Access for this Collection
3. Submit new Item to this Collection
4. Item will inherit restricted read policy
5. Delete Read Access group from Collection
6. Item now has NO read policies
Somewhat related to DS-2687