Gravity forms: checkboxes loaded by AJAX get unchecked on bad validation

Using Gravity Forms, the checkboxes are loaded by AJAX based on the value of a dropdown. All works fine. However, when the form is submitted and is invalid (eg. 1 required field left empty), all check boxes get unchecked.

I use $(document).on('gform_post_render', function( ) {} to re-load the checkboxes again after bad validation.

So the question is, how can I keep the checkboxes that where checked, checked?

I haven't found any example or solution concerning this issues. No errors in the console. Checkboxes loaded by Gravity forms stay checked on bad validation.

Any ideas?