Assume that you use MERGE statement to find out how many rows are not matched by source and store it in a local variable as follows:
@NotMatchedBySource INT = 0;
MERGE TargetTable USING SourceTable ON TargetTable.id = SourceTable.id
WHEN NOT MATCHED BY SOURCE THEN
UPDATE SET @NotMatchedBySource = @NotMatchedBySource + 1;
In this situation, you may notice that the value of @NotMatchedBySource after the MERGE statement may not reflect the actual number of rows that are not matched by source.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
This issue is fixed in the following cumulative updates for SQL Server
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