Nokia Music for Windows Phone - The Mixes feature disappeared after I logged in on MixRadio.

This may be because your Nokia Account is associated with a country where Mixes are not available. Mixes are available in Australia, Austria, Brazil, Canada, China, Finland, France, Germany, India, Indonesia, Ireland, Italy, Malaysia, Mexico, Netherlands, Norway, Poland, Portugal, Russia, Saudi Arabia, Singapore, South Africa, Spain, Sweden, Switzerland, Thailand, Turkey, UAE, UK, United States, Vietnam.