REGEXP with binary LHS: literal pattern works, user-variable / PS parameter raises ERROR 1300

Summary

On MariaDB 12.3.1 (rev 21a0714a), REGEXP behaves inconsistently when the left-hand side is a binary hex literal. A string-literal pattern works, but the identical pattern stored in a user variable or passed as a prepared-statement parameter raises ERROR 1300.

Minimal Reproducer

SET NAMES utf8mb3;

SELECT 0xE001 REGEXP '\\xE0\\x01';

-- 1

SET @pat='\\xE0\\x01';

SELECT HEX(@pat);

-- 5C7845305C783031

SELECT 0xE001 REGEXP @pat;

-- ERROR 1300 (HY000): Invalid utf8mb3 character string: 'E001'

PREPARE st FROM 'SELECT 0xE001 REGEXP ?';

EXECUTE st USING @pat;

-- ERROR 1300 (HY000): Invalid utf8mb3 character string: 'E001'

DEALLOCATE PREPARE st;

Expected

The user-variable and PS-parameter forms should behave like the literal form and return 1, not ERROR 1300.

Environment