DJM-900SRT Auto BPM problem

I have a problem with my DJM-900SRT's auto BPM detect.

I have my DJM connected to my Windows desktop via USB. I use it as an external soundcard. Audio is through the RCA outs into my speakers. All drivers are installed, firmware is the latest version.

Through USB, the mixer uses channel 1 (Deck 3) to transmit audio.

I noticed that with my effects channel selector set to 1, it fails to detect the BPM of whatever music is playing. If I do a quick switch of the channel input selector to Phono then back to USB, it displays the detected BPM.

Similarly, if I turn the effects channel selector to MIC then back to 4, 3, 2 or 1, it displays the detected BPM. If I turn the effects from 1 to MASTER it also displays the detected BPM.

However, if I leave my effects selector at MASTER the BPM auto detect works fine.

I have tried the CD/LINE inputs on each channel and the BPM auto detect works fine as it is. I can leave the effects channel selector on whatever channel audio input is coming to and auto BPM detect works fine.

Funnily enough, there was no such issue when I just got it (some 3 weeks ago). I only noticed it happening earlier. The only changes I've made to the mixer and my desktop is that I updated the firmware from v1.00 to the latest and installed rekordbox on my desktop.

I have also tried connecting it through USB to another laptop -- same issue.

In Ableton Live, I have my audio tracks routed to channels 1, 2 and 3 on the DJM. When I play a track on channel 1 and have my effects knob on channel 1, auto BPM detect fails to work. If I have a track playing on channels 2 or 3 and I have the effects selector knob on 2 or 3, auto BPM detect works just fine.

This is peculiar -- why is it only affecting channel 1; USB input, specifically?

Can anyone please help? It's bugging me.