I note that I ran into this loop, adding Containers along with Library, in versions 6.3.5 and 6.3.6. Thankfully, I found this post.
But the solution was more than simply 'Remove all Sandbox permissions'. I also had to:

1. Enable Global permissions. This covered all my browsers.
2. Click "Enable" for EACH browser down my list.
3. Approve of the resulting (all too familiar) requests for permissions: Library and Containers. Just once!
   Cookie is now sane again.