Feed style options appear to be read-only

Issue Summary

When I try to use the Fields view style, the options appear to be read only. The feed does not respond to/add any fields I add in the view's fields section. All RSS items rendered are a single "source" element and nothing more.

"Channel elements : core
Item elements : core
Other feed settings"

is all I get when I try to edit the feed style (gear icon next to "Style: RSS Feed - Fields". I can't change or do anything with those options because it's just text. Typically these areas expand and I can select/specify stuff but in this case it's just 3 lines of text. No expanding.