When creating a layergroup via the REST API, any style in a workspace is ignored in favour of the default style for the layer, as if an empty <style> element was submitted.
Steps to replicate:
1) Transfer/copy styles used by "tasmania" layer group into the "topp" workspace (suffix names with "_ws)".
2) Attempt to create layergroup with following command:
curl –u admin:XXXXXXXX –XPOST –H 'Content-type: text/xml' -firstname.lastname@example.org http://localhost:8080/geoserver/rest/layergroups
where tasmania.xml is the attached file.