Default value incorrect when editing pre-existing nodes

The Field Widget Settings screen has a feature that lets you select the Default Value.

When creating new nodes the Default Value seems to work ok.

However when the module was installed and the theme_field was added after there was already content on the site, and you go to edit an existing node that has not been saved with a theme_field value yet, the selected item is just the first one in the list (which tends to be "Bartik" if that theme is enabled) even if you've specified a separate theme on the field config.