The Fields tab displays a list of all of the fields, both native and custom, that exists in your Jira instance for classic-based projects only. It also contains additional columns of information that will assist Jira Administrators with knowing their exact purpose and usage in the instance.
...
| Warning |
|---|
IMPORTANT NOTE: Due to current the limitations of the Atlassian REST API, the list of fields will not include fields used on next-gen projects—only classic workflows. |
...