DDJ-400 Headphone cue/master mix out of sync - rekordbox buffer size < 1024 is broken

I often play in headphones only when recording a mix at home. So I need to cue and mix totally in the headphones, no amps, speakers, bluetooth device or anything else connected. However the headphone cue mix knob on my DDJ-400 does not work like it should because the master cue track is always out of sync with the headphones cued tracks.

My setup is a DDJ-400, MacOS Monterey, latest Macbook Pro with M1 chip, rekordbox 6.6.3.

You can reproduce this with a simple case using only 1 track:

1. Set your rekordbox buffer size to anything below 1024 samples, master output is set to DDJ-400 master, headphones output is set to DDJ-400 headphones.
2. Switch on master cue on the controller.
3. Start playing a track on channel 1, raise volume fader to max.
4. Switch on cue for channel 1.
5. Twist headphone mix knob clockwise to master - master track should be playing ok through the headphones.
6. Twist headphone mix knob anti-clockwise to cue - cued track should be playing ok through the headphones.
7. Now twist headphone mix knob to 12 o'clock. As it is the same track being cued as on master it should match perfectly and just be louder. Nope - the tracks are out of sync and sound like shoes in a dryer.

The same thing happens when you have 2 tracks playing. So if you want to beatmatch tracks in your headphones, you have to switch on cue for both channels then twist the mix knob all the way anti-clockwise to cue only. At least then they are in sync, but you don't have the ability to adjust the relative volumes in the cued mix because if you start twisting the knob towards the middle the master cue is out of sync.

After much stuffing around I have found why and it is the rekordbox buffer size setting. I had it set to 256 samples and decreasing it improved the problem somewhat, but even at 64 samples (1.5ms) I can still tell they are out. Increasing the samples makes things even worse until you hit 1024 samples and then magically they sync. Anything higher than 1024 and they stay in sync.

Conclusion: buffer size < 1024 is bugged as it fails to keep the headphone cue/master mix in sync.