CustomField extensions don't work correctly with CMS databases

1) Create new CustomField extension

2) Go to CMS database fields

3) Go to add field, your custom type will be shown from the available field types dropdown

4) After adding your field, it will not show in the field list

5) Going manually to the edit field page with the field id edited in the URL will say:

The custom field type YourFieldName does not exist, please select a valid field type.