DJM-900NXS2 Sample Rate Issue (use as sound card)

Hi Pioneer Folk,

I've got an issue getting the settings right to record at 24-bit/96kKz, which I believe should be the specification of the mixer and CDJ. Now before I start I realise that the human ear can't hear this, mine especially (I've got 16kHz max left in me) but this is more of a case of trying to identify whether I can use the DJM-900 as my hi-def soundcard, or whether I need to buy a separate one to meet specification.

As far as my understanding goes:

1. The CDJ-3000 supports 24-bit/96kHz WAV audio files.
2. The DJM-900NXS2 supports 24-bit/96kHz audio and can meet those specifications as a sound card.
3. ... Therefore I should be able to record 24-bit/96kHz audio via Audacity when I have my DJM connected as a soundcard (already configured through the settings utility)

So far so good, I think, although when I was recording some vinyl through the mixer-as-soundcard flow I noticed quite a harsh frequency cutoff almost exactly at 20kHz. With other audio interfaces I get some peaks above this to 21.5kHz approximately. Somewhat difficult to describe but the cutoff on the spectrogram felt quite severe when recording through the DJM so I ran some tests.

I downloaded the 96kHz "High Definition White Noise" test file from this site and confirmed the spectrogram that it does indeed touch 48kHz peak: https://www.audiocheck.net/testtones_highdefinitionaudio.php

Then I played this file through my CDJ-3000 which is connected via digital-out to my DJM-900NXS2 and then had the soundcard record the CDJ channel through Audacity at 24-bit/96kHz expecting to see essentially the same output as input. Based on specifications I'd expect the input and output to be approximately identical here, as far as I'm aware it's a fully digital chain and barring any differences due to faders and digital amps it should be 1:1, but indeed I get the 20kHz cutoff in the output recording like this:

The first two channels are the spectrograms L/R from the DJM soundcard and the bottom is the original file played through the CDJ-3000. You can see the frequency of the original file goes to 48kHz but there's a hard cutoff at 21kHz on the recording.

While I do still believe that this is an issue with my settings and/or setup I feel somewhat disappointed here as the specifications page does clearly state that it should support 24/96: https://www.pioneerdj.com/en-us/product/mixer/djm-900nxs2/black/specifications/

Has anyone got any ideas on what might be going wrong here? And any suggestions on what I can try?