Resolution: Not a bug
As per summary, custom multi line fields are not rendering URLs as hyper links in new JIRA issue view. System field like description still render URL fine.
In old JIRA issue view, URLs in old JIRA issue view are rendered as hyper links.
- Create a multi line custom field.
- Insert it into a screen of a project.
- Fill in any URLs into it.
URLs should automatically be rendered as hyper links.
The screenshot above is from the old JIRA issue view.
URLs are rendered as plain text.
Set Wiki Style as Renderer in Project Settings > Fields for the affected field. Once Wiki Style is used, the URL is correctly rendered as expected.