REGEXP_REPLACE converts utf8mb4 supplementary characters to '?'

The REGEXP_REPLACE function converts supplementary characters (4 byte utf8 encoding) into "?" when the charset is utf8mb4.

Due to JIRA not allowing supplementary utf8 characters in the issue description, I have used the CAST/UNHEX of the smiley character, but the actual value should be _utf8mb4'<emoji>'.

SELECT REGEXP_REPLACE(CAST(UNHEX('F09F9881') AS CHAR CHARACTER SET 'utf8mb4'), _utf8mb4'a', _utf8mb4'b') AS Text;

Expected output: "<emoji>"
Actual output: "?"