…the qs key when a (int) 0 form key is found.
TIL that when using the equal operator (==) to compare a string to an integer, the string is converted to an integer -- and not the other way around.
This means that if we stumble upon a (int) 0 form key in _market_source_form_find_element(), it will always match TRUE against the qs key we're trying to find. This will result in form submission chaos.
The fix, then, is to use the identical comparison operator, ===.