Tracker Motion Blur (possible bug)

Just after I managed to convince everyone around me that the motion blur in Fusion 8 is awesome... I can't seem to make it work anymore.
I'm using the built in motion blur from a tracker. Both the motion blur checkbox (from the tracker control area) and the MB (next to the playhead) are enabled.
Is there some other option that I might have accidentally switched off?

I'm using the built in motion blur from a tracker. Both the motion blur checkbox (from the tracker control area) and the MB (next to the playhead) are enabled.
Is there some other option that I might have accidentally switched off?