The Missing set to 0 will only be triggered when the number is being used in a calculations. That was the original purpose of the setting was to deal with a scenario where you have an empty row.
Changing that setting, and now you have the value calculated even with the empty row.
In your case, you could create a column of zeros and use that in the expression. Or use a column that has the length that you want and include that in the expression, where you multiply it by zero. That will return a value for PbO in every row as zero, even when it is empty.
FYI – There is also a quick way to fill all the empty values with 0.
Highlight all the columns in your group, including the ones that don’t have empty values.
Select Data > Change Empty rows to zero.
It will change the empty values to zero in all the rows where there is data, from the selected columns, up to the length of the longest column selected.
Does that help?