Just learned this. See if it works for you, and if it works on the FX9
On the FX6, switch ON "Proxy Record" and let it record its' MP4 file in the Sub folder. You should see 2 channels on that. But then look at the MXF file. Here, it now has 4 channels instead of 8.
Then turn OFF Proxy Record. For me, the MXF now has only 4 tracks, and it keeps that even when shifting recording modes.
Does that work for you???