I get the following error when I try to merge under a few conditions. Is someone able to explain why? Seems like it's a problem with SQL Server itself, but I wanted to post it here to confirm.

Attempting to set a non-NULL-able column's value to NULL.

1. The target table must be versioned
2. The history table must have a non-clustered index
3. Sufficient records must be inserted. In the example below, merging 2731 records fails, but 2730 merges in just fine

I am using SQL Server 2017 but have also observed it in Azure SQL

BEGIN TRANSACTION
SET XACT_ABORT ON;

CREATE TABLE RandomNumberHistory (Id INT NOT NULL, Number INT NOT NULL, [ValidFrom] DATETIME2 NOT NULL, [ValidTo] DATETIME2 NOT NULL);
CREATE TABLE RandomNumber (Id INT NOT NULL PRIMARY KEY CLUSTERED IDENTITY, Number INT NOT NULL, 
    ValidFrom DATETIME2 GENERATED ALWAYS AS ROW START, 
    ValidTo DATETIME2 GENERATED ALWAYS AS ROW END, 
    PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo),
)
WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.RandomNumberHistory));

/* Problematic index */
CREATE NONCLUSTERED INDEX [IX_RandomNumberHistory] ON RandomNumberHistory(Number);

CREATE TABLE #MergeTable (Number INT NOT NULL);

;WITH CTE AS
(
    SELECT ABS( CAST( CAST( NEWID() AS VARBINARY) AS INT ) ) % 100000 AS RandomNumber, 1 AS Counter
    UNION ALL
    SELECT ABS( CAST( CAST( NEWID() AS VARBINARY) AS INT ) ) % 100000 AS RandomNumber, Counter + 1 AS Counter
    FROM CTE
    WHERE CTE.Counter < 50000 /* Seems to fail starting at 2731 records */
)
INSERT INTO #MergeTable (CTE.Number)
SELECT RandomNumber
FROM CTE
OPTION (MAXRECURSION 0);

MERGE RandomNumber AS Target
USING (
    SELECT Number
    FROM #MergeTable
) AS Source
ON Target.Number = Source.Number
WHEN NOT MATCHED BY TARGET
THEN INSERT (Number) VALUES (Source.Number)
WHEN MATCHED THEN DELETE;
;

ROLLBACK TRANSACTION