I have installed the latest Extension Library and am building an Xpages app with Bootstrap. Select 2 is not included in the current Ext Lib. It is included in Xpages4Bootstrap.
could had be an AMD loading problem. When I look at Github github.com/select2/select2/blob/master/dist/js/select2.js I see an AMD check at the beginning of the code. Just remove the AMD if then else and keep the latest else clause or change define.AMD by false