Ths is an appeal to authority. It is a shortcoming of the product's design for it to autofill a hidden field.
How is it not a shortcoming? Any human looking at the page can immediately tell that this form field should not be autofilled.
Difficult problem to solve? Sure, doesn't make it any less of a shortcoming.
A product has a bug when it does not produce output in-line with its stated purpose and process. Nobody would use a refrigerator that suddenly forgets to keep food cold when a carton of milk is placed on the shelf.