UPPER() returns an empty string for U+0251 in uca1400 collations for utf8

CREATE OR REPLACE TABLE bad_case_folding

(

  code INT NOT NULL,

  c VARCHAR(32) CHARACTER SET utf8mb4 COLLATE uca1400_ai_ci NOT NULL

);

DELIMITER $$

FOR code IN 0..0x10FFFF

DO

  BEGIN

    DECLARE str TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_520_ci DEFAULT CHAR(code USING utf32);

    IF LENGTH(LOWER(str))=0 OR LENGTH(UPPER(str))=0 THEN

      INSERT INTO bad_case_folding VALUES (code, str);

    END IF;

  END;

END FOR;

$$

DELIMITER ;

SELECT HEX(code), HEX(LOWER(c)), HEX(UPPER(c)), c FROM bad_case_folding;

+-----------+---------------+---------------+----+

| HEX(code) | HEX(LOWER(c)) | HEX(UPPER(c)) | c  |

+-----------+---------------+---------------+----+

| 23A       |               | C8BA          | Ⱥ  |

| 23E       |               | C8BE          | Ⱦ  |

| 23F       | C8BF          |               | ȿ  |

| 240       | C980          |               | ɀ  |

| 250       | C990          |               | ɐ  |

| 251       | C991          |               | ɑ  |

| 252       | C992          |               | ɒ  |

| 26B       | C9AB          |               | ɫ  |

| 271       | C9B1          |               | ɱ  |

| 27D       | C9BD          |               | ɽ  |

+-----------+---------------+---------------+----+

CREATE OR REPLACE TABLE bad_case_folding

(

  code INT NOT NULL,

  c VARCHAR(32) CHARACTER SET utf8mb4 COLLATE uca1400_ai_ci NOT NULL DEFAULT ''

);

INSERT INTO bad_case_folding (code) VALUES (0x23A),(0x23E),(0x23F),(0x240),(0x250),(0x251),(0x252),(0x26B),(0x271),(0x27D);

UPDATE bad_case_folding SET c=CHAR(code USING utf32);

SELECT HEX(code), HEX(LOWER(c)), HEX(UPPER(c)), c FROM bad_case_folding ORDER BY code;

+-----------+---------------+---------------+----+

| HEX(code) | HEX(LOWER(c)) | HEX(UPPER(c)) | c  |

+-----------+---------------+---------------+----+

| 23A       |               | C8BA          | Ⱥ  |

| 23E       |               | C8BE          | Ⱦ  |

| 23F       | C8BF          |               | ȿ  |

| 240       | C980          |               | ɀ  |

| 250       | C990          |               | ɐ  |

| 251       | C991          |               | ɑ  |

| 252       | C992          |               | ɒ  |

| 26B       | C9AB          |               | ɫ  |

| 271       | C9B1          |               | ɱ  |

| 27D       | C9BD          |               | ɽ  |

+-----------+---------------+---------------+----+