All comments made before the post restriction changed to "Viewing and editing restricted" will be available to all user in all updates. This is only happening for blog post, and page restriction working as expected. Tested in version 5.9.1(customer's version and 6.1.3), same behavious can be seen.
h5. Step to reproduce:
User1 create public post and make comment #1
Restrict the post to user1 and make comment #2
Ask user2 to go to all updates: comment #1 visible but not comment #2
Remove all post restriction
User2 go to all updates again: both comments are visible, same goes to user1
Restrict the post to user1 again; User2 able to see the comment #1 in all updates