select2_extended_ajax_adapter.js

$.fn.select2.amd.define('select2/data/extended-ajax',['./ajax','../utils','jquery'], function(AjaxAdapter, Utils, $){

  function ExtendedAjaxAdapter ($element,options) {
    //we need explicitly process minimumInputLength value 
    //to decide should we use AjaxAdapter or return defaultResults,
    //so it is impossible to use MinimumLength decorator here
    this.minimumInputLength = options.get('minimumInputLength');
    this.defaultResults     = options.get('defaultResults');

    ExtendedAjaxAdapter.__super__.constructor.call(this,$element,options);
  }

  Utils.Extend(ExtendedAjaxAdapter,AjaxAdapter);
  
  //override original query function to support default results
  var originQuery = AjaxAdapter.prototype.query;

  ExtendedAjaxAdapter.prototype.query = function (params, callback) {
    var defaultResults = (typeof this.defaultResults == 'function') ? this.defaultResults.call(this) : this.defaultResults;
    if (defaultResults && defaultResults.length && (!params.term || params.term.length < this.minimumInputLength)){
      var processedResults = this.processResults(defaultResults,params.term);
      callback(processedResults);
    }
    else {
      originQuery.call(this, params, callback);
    }
  };

  return ExtendedAjaxAdapter;
});

For anyone coming to this with the same issue I had:

@vitorspencer thanks for the solution, working good 👍