The default GML link generated by the layer preview page uses WFS 1.0.0 as protocol and GML2 as output format, but this does not work for app-schema layers, since GML2 does not support complex features at all.
Clicking on the link results in a ServiceException response with the following error message:
Ideally, the generated GML link for app-schema layers should:
use WFS 1.1.0 as protocol
use either GML 3.1.1 or GML 3.2 as output format, depending on the schema
use the global OWS service instead of virtual services, to avoid problems accessing secondary namespaces