Workaround
Iurie Maxim: it works if in the querry are made separate requests for each typeName instead of one request for all typeNames. Most probably this approach has an issue as in this case typenames cant be provided as a parameter of the WFS request.
Iurie Maxim 21/4/2017: We solved this issue for downloading a dataset containing multiple type names, but the result has null values for namespaces. See problem 15.
Known bug from 2012 https://osgeo-org.atlassian.net/browse/GEOS-5512
Workaround Iurie Maxim: it works if in the querry are made separate requests for each typeName instead of one request for all typeNames. Most probably this approach has an issue as in this case typenames cant be provided as a parameter of the WFS request.