Looking at the API reference here, https://aerogear.org/docs/specs/aerogear-unifiedpush-rest/#-1637426673

A call to this endpoint will return an emtpy list even if there are windows variants

see the dev list for the description of the problem: http://aerogear-dev.1069024.n5.nabble.com/aerogear-dev-Possible-bug-in-getting-windows-variant-with-the-REST-API-tp12507.html