This one took a while to track down.

We were using a content type that had a couple field groups defined, with several fields in each, and some other fields that were not in any fieldgroup.

When used in a node widget, regardless of the settings, fields that were not put in a field group would never show up in the widget subform -- only fields that are in fieldgroups. The actual fieldgroups are ignored -- only fields inside a fieldgroup show up...

Workaround for us was to create a new fieldgroup and put the fields we wanted in the widget inside it.