I tested this code on 4.0.6 and 4.0.6 (only the releases and not the RC) and works well on 4.0.5. My usage of select2 is very simple:

$('#tags').select2({
   tags: true,
   createTag: function (params) {
      console.log("create tag", params)
     return params.term.match(/^[a-z0-9-_]+$/) ? {id: params.term, text: params.term, newTag: true} : null;
     }
});

the create tag function is fired for the first 2 characters of the tag and then stops for the rest.