Exporting 8 tracks MXF OP1a

Hello,

I'm trying to export a MXF with 8 audio tracks for broadcasting configured as follows:

CH 1 - Left Stereo

CH 2 - Right Stereo

CH 3 to CH 6 - Silence

CH 7 and CH 8 - Mono Mix

I've used my stereo mix panned left and right for the first two channels, and put down the mono mix in CH 7 and 8:

Despite this, the MXF is exported with only CH 1 and CH 2, and 6 silent channels. I've been trying all I can think of, but it just won't write CH 7 and 8:

I've moved the tracks on CH 7 and 8 to CH 3 and 4, but still won't write them in. The only way they get written in the MXF file is if I put them in CH 1 and 2.

Any thoughts?

Thanks,

Michele