I tested ISO 8601 behavior on PostgreSQL and MariaDB:
SELECT EXTRACT (WEEK from '2019-12-30'::date) => 1
SELECT WEEK('2019-12-30') => 52
I published result.
Andrew Hutchings answered:
Beat me to it �. We should maybe update the week doc to mention mode 3 is 8601
So, here is the ticket, to forget it.