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When the form is being validated it checks whether the option exists in the choices for the field, in this case:
queryset=AutoType.objects.filter(brand=Brand.objects.first())
It works only when you select an option from the first brand.
To make it work you need to update the choices for the auto_type field before it validates the data or it might be easier not to limit the choices of auto_type to any brand and then update the select element in JavaScript when the page loads to show only the options of the first brand.
Source:stackexchange.com