On a non-array document, '$[last]' makes JSON_EXTRACT return NULL but JSON_SET replace the whole document

-- '$[0]': the document is auto-wrapped, consistently

SELECT JSON_EXISTS('{"a":1}', '$[0]'),       -- 1

       JSON_SET('{"a":1}', '$[0]', 9),       -- 9

       JSON_INSERT('{"a":1}', '$[0]', 9);    -- {"a": 1}  (path exists: no-op)

-- '$[last]' on the same document

SELECT JSON_EXISTS('{"a":1}', '$[last]'),       -- 0

       JSON_SET('{"a":1}', '$[last]', 9),       -- 9   (whole document replaced)

       JSON_INSERT('{"a":1}', '$[last]', 9),    -- [{"a": 1}, 9]

       JSON_REPLACE('{"a":1}', '$[last]', 9),   -- {"a": 1}

       JSON_EXTRACT('{"a":1}', '$[last]');      -- NULL ('$[0]' returns the document)