It’s not 1Password fault, but poor design and implementation. :-)
This story could have easily been written about a user who fat fingered an extra 0 in the field.
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.
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.