Current behaviour

If you create an Epic Issue through JIRA + Create issue, all the fields you define for the issue creation screen will show up fine.

If on the other hand, you try to create an Epic in the Epic panel from inside an Agile board, only few fields will show up, and you will need to open the field configuration drop down to show the fields you need, changing the selection to All fields.

Expected behaviour

Use the screen you've configured to create Epics in JIRA Agile as well, not a different screen.