Steps to reproduce:
- Use a Zen-based theme on the content creation page.
- Go to the node form. The "Edit summary" link will not be present for the above field - instead, the body and summary will display separately one above the other.
Credit should go to mndonx for discovering the root cause.
I have a patch which fixes the problem by simply not swapping out the field type class in the particular case of a textarea with summary. It's not an ideal fix; however, considering that this is already exactly what happens for the standard Drupal body field (that field does not have its name begin with "field_" so none of the code in zen_form_node_form_alter() ever ran for it anyway) it seems reasonable to fix it by making other fields consistent. The Body field is used much more often in practice than a second "body-like" field, so if no one ever complained about the field type classes being inconsistent for the Body field they aren't too likely to complain about it here either.