Receiving different output for column SUMs in DB2

I have the following table in DB2:

Below query would return the sum of value periods for each Period Type, grouping them into Product Code

SELECT
    D.DMPROD,
    Sum(D.DMVL01 + D.DMVL02 + D.DMVL03 + D.DMVL04 + D.DMVL05 + D.DMVL06) AS Total
FROM
        DWM D
WHERE
        D.DMYEAR IN (2022)
    AND D.DMPTYP = 'M'
    AND D.DMTYPE IN ('RTNQTY','SLSQTY')
GROUP BY
        D.DMPROD
ORDER BY
    1;

Initial output:

Then I changed this to work with a subquery thanks to feedback from other members. This is the preferred format:

SELECT
    D.DMPROD,
    (SLSQTY + RTNQTY) AS Total
FROM
    (
    SELECT
        D.DMPROD,
        Sum(CASE WHEN D.DMTYPE = 'RTNQTY' THEN Total END) AS RTNQTY,
        Sum(CASE WHEN D.DMTYPE = 'SLSQTY' THEN Total END) AS SLSQTY
    FROM
        (
        SELECT
            D.*,
            (D.DMVL01 + D.DMVL02 + D.DMVL03 + D.DMVL04 + D.DMVL05 + D.DMVL06) AS Total
        FROM
            DWM AS D
    ) AS D
    WHERE
        D.DMYEAR IN (2022)
        AND D.DMPTYP = 'M'
    GROUP BY
        D.DMPROD
) AS D
ORDER BY 1;

but notice the sums are not the same (the initial output was correct):

Where did I make the mistake with the sums?

These expressions:

    Sum(CASE WHEN D.DMTYPE = 'RTNQTY' THEN Total END) AS RTNQTY,
    Sum(CASE WHEN D.DMTYPE = 'SLSQTY' THEN Total END) AS SLSQTY

can return NULL if no rows for the DMTYPE are in the result. That, in turn, affects this calculation:

(SLSQTY + RTNQTY) AS Total

The simplest solution is to add ELSE 0:

    Sum(CASE WHEN D.DMTYPE = 'RTNQTY' THEN Total ELSE 0 END) AS RTNQTY,
    Sum(CASE WHEN D.DMTYPE = 'SLSQTY' THEN Total ELSE 0 END) AS SLSQTY

This avoids the NULL value, so the total calculation does not turn into NULL.