STR_TO_DATE returns NULL only when en_US locale is omitted under utf8mb4_bin

MariaDB 12.3.3-MariaDB-ubu2404 returns NULL when STR_TO_DATE parses a lowercase English month under utf8mb4_bin using the session's en_US locale. Supplying the same locale explicitly returns 2026-03-01.

Reproducer

Run these statements in one MariaDB session:

SET NAMES utf8mb4;

SET lc_time_names='en_US';

SELECT 'bin_lower', STR_TO_DATE(_utf8mb4'1 march 2026' COLLATE utf8mb4_bin,'%d %M %Y'), STR_TO_DATE(_utf8mb4'1 march 2026' COLLATE utf8mb4_bin,'%d %M %Y','en_US');

SHOW WARNINGS;

SELECT 'ci_lower', STR_TO_DATE(_utf8mb4'1 march 2026' COLLATE utf8mb4_general_ci,'%d %M %Y'), STR_TO_DATE(_utf8mb4'1 march 2026' COLLATE utf8mb4_general_ci,'%d %M %Y','en_US');

SELECT 'bin_title', STR_TO_DATE(_utf8mb4'1 March 2026' COLLATE utf8mb4_bin,'%d %M %Y'), STR_TO_DATE(_utf8mb4'1 March 2026' COLLATE utf8mb4_bin,'%d %M %Y','en_US');

Decisive observation

The first value after each label uses the session locale implicitly; the second passes en_US explicitly.

bin_lower  NULL        2026-03-01

Warning    1411        Incorrect datetime value: '1 march 2026' for function str_to_date

ci_lower   2026-03-01  2026-03-01

bin_title  2026-03-01  2026-03-01

The failure is limited to the combination of the lowercase month, binary collation, and omitted locale argument in this case. The case-insensitive-collation control and title-case control both produce the same date with omitted and explicit locales.

Semantic mismatch

With lc_time_names='en_US', omitting the optional locale and explicitly passing en_US select the same locale. Both calls receive identical text and format and should therefore parse the date as 2026-03-01. Instead, only the omitted-locale call returns NULL and raises warning 1411.

This occurs when STR_TO_DATE parses a lowercase localized month under a binary collation while relying on lc_time_names instead of an explicit locale argument.