I am trying to implement SCD type 2 using MERGE clause in SQL SERVER. I have written the following code:

INSERT INTO
  Dim_Customer (
    Customer_Internal_ID,
    Customer_Name,
    Price_Level,
    INS_TS,
    Is_active
  )
SELECT
  Internal_ID,
  Name,
  Price_Level,
  GETDATE(),
  1
FROM
  (
    MERGE Dim_Customer AS T USING (
      SELECT
        Internal_ID,
        NULLIF(Name, '') Name,
        NULLIF(Price_Level, '') Price_Level
      FROM
        Stg_Customer
    ) AS S ON T.Customer_Internal_ID = S.Internal_ID
    WHEN NOT MATCHED THEN
    INSERT
      (
        Customer_Internal_ID,
        Customer_Name,
        Price_Level,
        INS_TS,
        Is_active
      )
    VALUES
      (
        S.Internal_ID,
        S.Name,
        S.Price_Level,
        GETDATE(),
        1
      )
      WHEN MATCHED
      AND T.Is_active = 1
      AND (
        ISNULL(T.Customer_Name, '') <> ISNULL(S.Name, '')
        OR ISNULL(T.Price_Level, '') <> ISNULL(S.Price_Level, '')
      ) THEN
    UPDATE
    SET
      T.Is_active = 0 OUTPUT S.Internal_ID,
      S.Name,
      S.Price_Level,
      $ACTION AS Merge_Action
  ) MRG
WHERE
  MRG.Merge_Action = 'UPDATE';

For few records it is detecting the changes, but instead of updating with the new value it is inserting the same values again and marking Is_active = 1.
Staging has everything in VARCHAR.
Price_Level has values like following:
Tier 3 (38% Discount)
Tier 4 (35% Discount) etc.

I tried to implement SCD type 2 using MERGE clause, but it is not working for few records.