DDJ-FLX10 mono split issue

Hi

I'm using an FLX10 with Rekordbox 6.7.3. on a fast Windows 11 laptop.

I've been trying to use the Mono Split functionality and have selected it in the mixer tab.  My understanding of that functionality is that it sends (i) the sound of any channel that is cued to the left earphone and sends (ii) anything that's outputting to the master to the right headphone.

I have no issue with (i), but the master sound isn't being outputted to the right headphone.  Nothing is.

I've tried trouble-shooting.  My headphones are fine (I can tick invert the LR audio and I have the opposite issue.  There is output to the master (I have monitors connected by XLR).  I've rebooted.  The headphone mix is in the middle; I've tried turning that left to right, but can't get any output in my right earphone.

I'd much appreciate any advice anyone can offer.

Thanks!