On BFD’s dashboard, on the right edge, there are 3 labels: Off, Groove and track.
When set to off, bfd will play what’s written in the parts of the instrument track in Cubase. (I think that’s what you want)
When set to Groove, BFD will play the selected groove of the groove editor. (Of course, it will still try to play what’s on the track, resulting in double sounds)
When set to track, BFD will play the track formed within its own groove editor. (I also believe you can get double drums this way)
So, just turn it to “off”, and start from there, because there could be more at play here, I’m just stating the most obvious first.