I have created a SQL query which will show the current sales and sales from previous year with the LAG window function, this per customer, product, year and month. A product is not always sold each year and I know that LAG does not care about missing years, therefore the CASE statement, based on this stackoverflow post: LAG function alternative. I need the results for the missing year in between

Now the problem is that on detailed level the previous year is correct with the LAG function but when it is grouped on year it does not match anymore (See table results below)

These are my tables:

DIM_CUSTOMER

DECLARE @DIM_CUSTOMERS TABLE([BusinessKey] INT,[Customer] NVARCHAR(255))
 
INSERT INTO @DIM_CUSTOMERS
VALUES 
(10000, 'Kevin N.V.'),
(10001, 'V.Z.W. Frederik'),
(10002, 'Klaas N.V.')
SELECT * FROM @DIM_CUSTOMERS

DIM_PRODUCTS

INSERT INTO @DIM_PRODUCTS
VALUES
(9000, 'PH114'),
(9001, 'PH272'),
(9002, 'PH878'),
(9003, 'PH900')
SELECT * FROM @DIM_PRODUCTS

DIM_DATES

DECLARE @DIM_DATES TABLE([BusinessKey] INT, [Year] INT, [Month] INT, [YearMonth] INT, [YearMonthText] NVARCHAR(20))

INSERT INTO @DIM_DATES
VALUES
(202201, 2022, 1, 202201, '2022.01'),
(202202, 2022, 2, 202202, '2022.02'),
(202203, 2022, 3, 202203, '2022.03'),
(202204, 2022, 4, 202204, '2022.04'),
(202205, 2022, 5, 202205, '2022.05'),
(202206, 2022, 6, 202206, '2022.06'),
(202207, 2022, 7, 202207, '2022.07'),
(202208, 2022, 8, 202208, '2022.08'),
(202209, 2022, 9, 202209, '2022.09'),
(202210, 2022, 10, 202210, '2022.10'),
(202211, 2022, 11, 202211, '2022.11'),
(202212, 2022, 12, 202212, '2022.12'),
(202301, 2023, 1, 202301, '2023.01'),
(202302, 2023, 2, 202302, '2023.02'),
(202303, 2023, 3, 202303, '2023.03'),
(202304, 2023, 4, 202304, '2023.04'),
(202305, 2023, 5, 202305, '2023.05'),
(202306, 2023, 6, 202306, '2023.06'),
(202307, 2023, 7, 202307, '2023.07'),
(202308, 2023, 8, 202308, '2023.08'),
(202309, 2023, 9, 202309, '2023.09'),
(202310, 2023, 10, 202310, '2023.10'),
(202311, 2023, 11, 202311, '2023.11'),
(202312, 2023, 12, 202312, '2023.12'),
(202401, 2024, 1, 202401, '2024.01'),
(202402, 2024, 2, 202402, '2024.02'),
(202403, 2024, 3, 202403, '2024.03'),
(202404, 2024, 4, 202404, '2024.04'),
(202405, 2024, 5, 202405, '2024.05'),
(202406, 2024, 6, 202406, '2024.06'),
(202407, 2024, 7, 202407, '2024.07'),
(202408, 2024, 8, 202408, '2024.08'),
(202409, 2024, 9, 202409, '2024.09'),
(202410, 2024, 10, 202410, '2024.10'),
(202411, 2024, 11, 202411, '2024.11'),
(202412, 2024, 12, 202412, '2024.12')
SELECT * FROM @DIM_DATES

FACT_SALES

DECLARE @FACT_SALES TABLE([ID] INT, [FK_Product] INT, [FK_Customer] INT, [FK_Date] INT, [Sales] FLOAT)

INSERT INTO @FACT_SALES
VALUES
(1, 9000, 10000, 202303, 90.48),
(2, 9000, 10000, 202304, 20.40),
(3, 9002, 10000, 202305, 250.85),
(4, 9002, 10000, 202303, 100.50),
(5, 9000, 10000, 202403, 38.40),
(6, 9000, 10000, 202406, 474.50),
(7, 9001, 10000, 202403, 128.60),
(8, 9001, 10000, 202404, 144.97),
(9, 9000, 10002, 202303, 199.60),
(10, 9001, 10002, 202302, 58.97),
(11, 9001, 10002, 202402, 40.88),
(12, 9001, 10000, 202203, 14.5)
SELECT * FROM @FACT_SALES

MY_ATTEMPT

;WITH CustProdYears 
    AS(
        SELECT DISTINCT 
            d.[Year] as SaleYear, s.FK_Product, s.FK_Customer
            FROM @FACT_SALES   s     
            JOIN @DIM_DATES d on s.FK_Date = d.BusinessKey
    )
, CustomerSales 
    AS (
        SELECT cpy.FK_Customer, cpy.FK_Product, d.YearMonthText, s.[Sales], d.year, d.month
            FROM CustProdYears cpy 
            JOIN @DIM_DATES d on cpy.[SaleYear] = d.[Year]
            LEFT JOIN @FACT_SALES s 
                on s.FK_Customer = cpy.FK_Customer
                and s.FK_Product = cpy.FK_Product
                and s.FK_Date = d.BusinessKey
    )
SELECT b.Year,
        SUM(b.[Sales]) AS [Sales],
        SUM(b.[SalesLastYear]) AS [SalesLastYear]
FROM
(
    SELECT *,
        LAG(a.year, 1, 0) OVER (PARTITION BY a.Customer,  a.Product, a.month ORDER BY a.year) AS [PreviousYear],
        CASE WHEN 1 = a.year - LAG(a.year, 1, 0) OVER (PARTITION BY a.Customer,  a.Product, a.month ORDER BY a.year)
            THEN LAG(a.[Sales], 1, 0) OVER (PARTITION BY a.Customer,  a.Product, a.month ORDER BY a.year)
            ELSE 0 END AS [SalesLastYear]
    FROM
    (
    SELECT 
          Customer, Product, Year, Month
        , [Sales] = ISNULL([Sales], 0)
        FROM @DIM_CUSTOMERS c 
            LEFT JOIN CustomerSales s ON c.BusinessKey = s.FK_Customer
            LEFT JOIN @DIM_PRODUCTS p on s.FK_Product = p.BusinessKey
            WHERE Customer = 'Kevin N.V.'

    ) a
    --ORDER BY a.Customer, a.Product, a.year, a.month
) b
GROUP BY b.Year
ORDER BY b.Year

My result:

Expected result:

Is there a way to fix this issue?