MySql TIMESTAMPDIFF in Update fails

I'm using MariaDB 10.4.25 and am trying to update some data in a WordPress table, but getting an error.

I can run the SELECT query without a problem, but then when I run the UPDATE, it throws a "Incorrect datetime value: month" error.

SELECT QUERY:

SELECT
    pm.post_id,
    pm.meta_key,
    pm.meta_value AS billing_interval,
    p1.meta_value AS schedule_start,
    p2.meta_value AS schedule_end,
    TIMESTAMPDIFF(YEAR, p1.meta_value, p2.meta_value) AS difference
FROM    wp21_postmeta AS pm
JOIN    wp21_posts AS p ON pm.post_id = p.ID
LEFT JOIN    wp21_postmeta AS p1 ON pm.post_id = p1.post_id AND p1.meta_key = '_schedule_start'
LEFT JOIN    wp21_postmeta AS p2 ON pm.post_id = p2.post_id AND p2.meta_key = '_schedule_end'
WHERE
    pm.meta_key = '_billing_interval'
    AND p.post_type = 'shop_subscription'
    AND pm.meta_value != TIMESTAMPDIFF(YEAR, p1.meta_value, p2.meta_value)
    AND TIMESTAMPDIFF(YEAR, p1.meta_value, p2.meta_value) != 0;

which correctly displays the values.

5319233 _billing_interval   1   2024-01-02 20:10:53 2026-01-02 20:10:53 2
5319234 _billing_interval   1   2024-01-02 20:10:53 2027-01-02 20:10:53 3
5319256 _billing_interval   1   2024-01-03 20:48:04 2026-01-03 20:48:04 2
5319257 _billing_interval   1   2024-01-03 20:48:04 2027-01-03 20:48:04 3
5319262 _billing_interval   1   2024-01-04 19:05:46 2026-01-04 19:05:46 2
5319263 _billing_interval   1   2024-01-04 19:05:46 2027-01-04 19:05:46 3
5319238 _billing_interval   1   2024-01-02 20:12:07 2026-01-02 20:12:07 2
5319239 _billing_interval   1   2024-01-02 20:12:07 2027-01-02 20:12:07 3

However, when I try the corresponding UPDATE I get the error:

UPDATE    wp21_postmeta AS pm
JOIN    wp21_posts AS p ON pm.post_id = p.ID
LEFT JOIN    wp21_postmeta AS p1 ON pm.post_id = p1.post_id AND p1.meta_key = '_schedule_start'
LEFT JOIN    wp21_postmeta AS p2 ON pm.post_id = p2.post_id AND p2.meta_key = '_schedule_end'
SET pm.meta_value = TIMESTAMPDIFF(YEAR, p1.meta_value, p2.meta_value)
WHERE
    pm.meta_key = '_billing_interval'
    AND p.post_type = 'shop_subscription'
    AND pm.meta_value != TIMESTAMPDIFF(YEAR, p1.meta_value, p2.meta_value);

Error code: 1292. Incorrect datetime value: 'month'

The error appears to be from the TIMESTAMPDIFF in the WHERE clause, since if I remark out that line, no error occurs. But I'm not clear why it works fine in the SELECT, but not the UPDATE. I also tried to convert the meta_values to datetime, but same error.