The following query on the attached table & data yields different results in MySQL 5.5 & MariaDB 5.5:

SELECT id, date_i, date_ontop, date_until
FROM
(
SELECT id, date_i, date_ontop, date_until
FROM news
WHERE
date_i <= CURRENT_TIMESTAMP
AND
date_ontop >= CURRENT_TIMESTAMP
ORDER BY date_i DESC
) AS ontop

UNION

SELECT id, date_i, date_ontop, date_until
FROM
(
SELECT id, date_i, date_ontop, date_until
FROM news
WHERE
date_i <= CURRENT_TIMESTAMP
AND
( date_until >= CURRENT_TIMESTAMP OR date_until IS NULL)
AND
( date_ontop IS NULL OR date_ontop < CURRENT_TIMESTAMP )
ORDER BY date_i DESC
) AS therest
LIMIT 0, 18

In MariaDB 5.2 it gives the same results as MySQL 5.5.

It may be related to Bug#1010116 because if I switch optimizer_switch='derived_merge=off' it give the correct results.