JSON modal field is not updated on fetch

I have created JSON modal with 2 fields “template” (select) and “message text” (textarea). I need to select template and set (using fetch) “message text” according to template.

This work only one time. When i select template first time - i get fetch request, return message text, and it set into textarea field.

But when i select template second time - i get fetch request, return message text, but textarea field stay unchanged.

Is there way to change textarea field on every fetch request ?