Empty dataset even though the Log View shows results when querying IP adresses

I struggle to understand why this dataset query shows no result on my FortiAnalyzer instance:

SELECT
  dstport,
  srcip,
  dstip
FROM
  $log
WHERE
  $filter
  AND ipstr(dstip) IN ('172.31.11.80', '172.31.11.83')
GROUP BY
  srcip,
  dstip,
  dstport
ORDER BY
  dstport,
  srcip

To be sure, I am getting results if in the Log View I search for: 

 dstip=172.31.11.80 or dstip=172.31.11.83

Any hint?