Unable to set course category to root level

As illustrated in the first attached image I'm unable to set a course category to root level via "Add to root" in the batch processing in the back-end or when editing a specific category and attempting to set it to root in the "Parent" drop-down ("Add to root" is not an option).

However, as you can see from the second attached image, it apparently was possible at one time to do so because this site has numerous "root-level" course categories.