I don’t think there are rules for this case and you cannot depend on a particular outcome.
If you’re after a specific row, say the latest one, you can use apply
, like:
UPDATE a
SET a.Value = b.Value
FROM tableA AS a
CROSS APPLY
(
select top 1 *
from tableB as b
where b.id = a.id
order by
DateColumn desc
) as b