According to Jira's info page you should be able to change custom fields shown:
"To choose which custom fields appear in the new issue view, head to Project settings > Screens, expand the appropriate screen scheme, select the "View issue" screen to configure it. Field order isn't configurable in the new issue view just yet."
However, when I do alter the View Issue Screen scheme, making sure the custom field I want to appear, I can't seem to get my custom fields to appear, such as a multi-option custom field. It is not sorted at the bottom either so there isn't some kind of cap it might have reached.
I'm on Jira Cloud / Ondemand