Login on Syncope console and create (or edit) an user and assign to him a group that has type extensions. You should see a new accordion in plain attributes tab named with the name of the group. Then edit values of the attributes in that accordion and save. Re-edit the user and you'll see that values are not shown in the form.
But reading user through http://localhost:9080/syncope/rest/users/[username] memberships plain attrs have values. The problem is related only to view.