【Bug?】The behavior of the UPDATE statement using the CASE clause is wrong

[MarkMpn]

Discussed in #313

^{Originally posted by hiroyuki0415 June 26, 2023}
I have the following data:

	crf3c_rank0_3id	crf3c_integer_column
	2f0384fb-5b14-ee11-9cbe-002248627e12	1
	63aecf0d-5c14-ee11-9cbe-002248627e12	1
	13a93ef8-5b14-ee11-9cbe-002248627ef1	1
	b51e0406-5c14-ee11-9cbe-002248627ef1	1
	08a15f0d-5c14-ee11-9cbe-002248627ef1	1

After executing the following UPDATE, all the values ​​in the [crf3c_integer_column] column are NULL.
Isn't this a bug?

UPDATE r
SET r.crf3c_integer_column = (
		CASE 
			WHEN r.crf3c_integer_column IS NULL
				THEN NULL
			ELSE r.crf3c_integer_column + 1
			END
		)
FROM crf3c_rank0_3 AS r
WHERE r.crf3c_rank0_3id IS NOT NULL;

The result is below:

	crf3c_rank0_3id	crf3c_integer_column
	2f0384fb-5b14-ee11-9cbe-002248627e12	NULL
	63aecf0d-5c14-ee11-9cbe-002248627e12	NULL
	13a93ef8-5b14-ee11-9cbe-002248627ef1	NULL
	b51e0406-5c14-ee11-9cbe-002248627ef1	NULL
	08a15f0d-5c14-ee11-9cbe-002248627ef1	NULL

Thanks in advance for your support.