external checks scheduled to run once a week no longer working

We have a setup, described in ZBX-3961, and it has stopped working recently after an upgrade.

To recap, we have an external item with a flexible interval "1,12:45-12:46" with a delay of 60 seconds. So the item should run once a week on Mondays around that time.

It used to work fine, but on March 17, the server was upgraded to r26149. After that, the item was checked once, on Monday, March 19, but never since then.