Extensible forms broken

Extensible forms and/or extensible models are broken in the following way:

• Fields are correctly added to the Layout
• The respective model fields are available on the model
• The model form below the extensible form does not see the fields in the model, so the form fields never get created. Thus , when rendering the layout, the fields are not found on the form.

    raise KeyError(
KeyError: "Key 'show_in_timetables' not found in 'AnnouncementForm'. Choices are: description, groups, persons, title, valid_from, valid_from_date, valid_from_time, valid_until, valid_until_date, valid_until_time."

I have checked the import order and that when forms is imported, the extended model fields are correctly available and vidisble.