Unable to revert sorting method of field values from "Sort by Name" to "Sort Manually"

This relates to sorting the list of values or choices for both the pre-build field `subsystem` as well as other custom fields. It is *not* related to sorting how fields are displayed on an issue or report.

Initially, the `Sort By` setting for the subsystem field was set to manual. This provided a handlebar on the left of the value which I could drag/drop to rearrange the list. Upon switching to Sort Manually, a dialog modal is presented with the following text: "Change Sort Order - When you confirm this action, the manual sort order is discarded. If you revert to sorting values manually, you have to reapply the custom order."

Clicking ok causes the list to be sorted alphabetically. However, it seems there is no way to undo this choice and revert back to manual sorting. After making the change, there is no longer any other option in the 'Sort By' drop-down menu.

Steps to reproduce:

1. Projects --> [project_name] --> Fields
2. Add field to project
3. Type = ownedField, Set of Values = new Set
4. Add several new values
5. Use handlebars on left to arrange in custom order
6. Change from `Sort Manually` to `Sort By Name`
7. Read the warning and hit ok
8.  Values are now sorted alphabetically
9. Option to change back to `Sort Manually` is no longer present. Only choice is `Sort by Name`