I am not sure, if this is a bug or behaviour per definition.
If I have two styles "test". One is a global style and one a standard workspace style. Then the request for the global style delivers always the workspace one. At least
includes a workspace definition. The same occures within the style gui of geoserver. The global has a workspace.
As I said, that does only occur if the workspace is standard. If it isnt, Geoserver distinghuishes both styles correctly.