Apparently the NMBS/SNCB website now makes a difference between “Brussels-Midi” and “Brussels-Midi Eurostar”; same station, different track.
Due to this change, iRail was no longer able to provide any scheduling information which included Brussels-Midi.
I’ve made a quick and dirty work around (by including “+ Eurostar” in the drop down menu), solving this bug.
However, please, NMBS/SNCB, please please please provide an API or report such changes to web developers.
As it is now, if you manually fill in Brussels-Midi and do not add the Eurostar text, the iRail website will not be able to to look up the requested information.
Thanks to Stéphane for the bug report.