The function (RemoveFromRole in UserManager) to remove user from role is seriously buggy! If users has many roles the function seems to remove a random role but fortunately correct user.
Has anyone else experienced this?
There is a bug tracking this issue http://aspnetidentity.codeplex.com/workitem/2069 As a workaround you can remove all roles for the user and add the ones for now. It is not the best solution but it will work