Missing my custom ajax field values by image field in node form alter
In your form an AJAX call is made when your car_brand element is changed but also when upload image button is clicked. Your problem is the AJAX call that is made when upload button is clicked. In that call car_info_form_car_info_node_form_alter() is called, and probably is modifying the allowed options array for the model select element in a wrong way.

In your form altering function check for the element that fired the AJAX call  modifying the select element options only when your first select (brands). You can check for that element in the $form_state['triggering_element'].