Oddly, in the backend of PayPlans you can not assign a plan to a group when the group is set to Not Visible.
Additionally, if you set a group to Not Visible and then go into a plan and make changes to it, and save the plan, the group association is removed during the save process.
My opinion is that it should NOT disassociate, and in the backend the admin should still be able to assign a plan to a group.
James