Thats because of the chosen.js - i had the same question a few weeks ago :

Update List field rendering after ajax update

You can simply call:

jQuery('#jform_identificator').trigger("chosen:updated");

after you changed the options, this triggers the update.

kind regards Marco

Thats because of the chosen.js - i had the same question a few weeks ago :

Update List field rendering after ajax update

For chosen you can call:

jQuery('#jform_identificator').trigger("chosen:updated");

after you changed the options, this triggers the update.

Edit: in Joomla! V3.x and old version of chosen is used and the trigger is:

jQuery('#jform_identificator').trigger("liszt:updated");

kind regards Marco