I got it to work by issuing "Cease Fire", then "Open Fire" again.
Update: Nope, didn't work. What worked, however, was manually targeting a salvo. After the first one disappeared, they automatically switched to new salvos.
I think clicked "Fire at will" at some point. Since there is no indication of FC status set to that, I don't know if it still applied then.
Also, they pick stupid targets.
Next edit: This time I got it to work. I manually targeted one salvo out of 8 before it got into range. They fired, I received reports for each shot (which you apparently don't get if they kindly decide to auto-fire) and they switched to new targets when appropriate.
