MariaDB returns inconsistent results for a simple CASE predicate in WHERE when its WHEN operand contains double negation on a string column

Description

The same simple CASE expression evaluates to 1 when selected as a value, but the row is excluded when the expression is used as a WHERE condition. The query that counts rows matching the predicate returns 0, while counting the true values of that same predicate in the select list returns 1. I verified the discrepancy repeatedly on MariaDB 12.3.2.

Reproduction

CREATE DATABASE mariadb_case_when_repro;

USE mariadb_case_when_repro;

CREATE TABLE t (

    a VARCHAR(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin,

    b VARCHAR(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin

);

INSERT INTO t VALUES ('x', 'y');

SELECT

    a,

    b,

    CASE a WHEN NOT NOT b THEN 1 ELSE 0 END AS case_value,

    IF(CASE a WHEN NOT NOT b THEN 1 ELSE 0 END, 1, 0) AS projected_truth

FROM t;

SELECT COUNT(*) AS where_count

FROM t

WHERE CASE a WHEN NOT NOT b THEN 1 ELSE 0 END;

SELECT SUM(IF(CASE a WHEN NOT NOT b THEN 1 ELSE 0 END, 1, 0))

       AS projected_count

FROM t;

Expected Result

The first query returns case_value = 1 and projected_truth = 1. Since the predicate evaluates to true for the only row, both counts should be 1:
where_count = 1
projected_count = 1

Actual Result

On MariaDB 12.3.2:
case_value = 1
projected_truth = 1
where_count = 0
projected_count = 1

Thus, the row is returned as a true predicate in the projection, but is filtered out when the same predicate is used in WHERE.

Cross-Database Check

With the same table and statements, MySQL 8.0.46, MySQL 8.4.7, Percona Server 8.4.11, and TiDB 8.5.8 return 1 for both counts.