Exporting multitrack submaster but some tracks export blank

Hello

Premiere Pro Version 15.1.0 (Build 48)

Media Encoder Version 15.1 (Build 42)

I know I'm probably staring at the problem and will facepalm when I see the answer but I'm trying to export a 12 audio track MXF OP1a and, while it's creating a 12 track file only 8 tracks have audio. the bottom 4 tracks are blank.

Here are my Sequence Settings:

Here are my output settings:

Here is my timeline:

And here is my timeline with the output file dragged in to check (note tracks 9-12 have been created but the audio is blank):

Like I said, I know I've probably just got some setting incorrect but I can't, for the life of me, figure it out.

'Any thoughts?

(Note: I will be cross-posting this to the Premiere Pro forum)