Post appearing in category archive when it shouldn’t

Some additional info, after further research with Query Monitor. The issue appears to come down to this query:

SELECT t.*, tt.*
FROM cpdm_terms AS t
INNER JOIN cpdm_term_taxonomy AS tt
ON t.term_id = tt.term_id
WHERE tt.taxonomy IN ('category')
AND t.term_id IN ( 185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,806,9008 )

Of those category IDs, only 185-193 are actually children of 9008, the rest are other categories (none of which are related to 9008 in any way). This is, however, the correct quantity of child categories. In reality, 177-193 and 801 are the children of 9008. So, somehow, it’s choosing to pull the wrong categories in…

Just tried it in vanilla Twenty Twenty-One, same basic issue:

SELECT SQL_CALC_FOUND_ROWS cpdm_posts.ID
FROM cpdm_posts
LEFT JOIN cpdm_term_relationships
ON (cpdm_posts.ID = cpdm_term_relationships.object_id)
WHERE 1=1
AND ( cpdm_term_relationships.term_taxonomy_id IN (185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,806,9008) )
AND cpdm_posts.post_type = 'post'
AND (cpdm_posts.post_status = 'publish'
OR cpdm_posts.post_status = 'private')
GROUP BY cpdm_posts.ID
ORDER BY cpdm_posts.post_date DESC
LIMIT 0, 10