Potentiometers ground issue(?) when audio out socket isn't used

I made this oscilloscope device using the Daisy Seed. There’s a thread about its development here (click through to see code):

All that’s on it is audio in and out sockets, an OLED screen, and two potentiometers for adjusting the horizontal and vertical scale of the oscilloscope. The audio I/O is just meant to be a bypass, but I also intended for the device to be used with input only. However, when the output socket is not being used the potentiometers behave in a manner consistent with ground issues (such as a fault between AGND and DGND). Here is a video of the problem in action (no sound):

When I unplug the output cable, the potentiometer positions are “ignored”. Also, when the sleeve/ground of the output socket is disconnected, same problem. If at least one of the stereo split cables on my output cable is plugged into my mixer it works (even if the mixer is unpowered), but it doesn’t work if both are removed. Additionally, it doesn’t work if say, headphones are plugged into the output.

Everything also works fine if the input socket is not used, as long as the output socket is.

One last thing as I was trying to measure values while testing. Weirdly, regardless of configuration, Serial seems unable to read the potentiometer values. Even if the configuration is “right” and the potentiometers work correctly in practice, Serial.println(knob) only reads out “194” (or “192” in the case of the other knob).

Here is a rudimentary wiring diagram of my device:

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Just to confirm - are you using stereo jacks? NOT shorting jacks?

I’m using stereo jacks, these ones to be specific https://www.amazon.com/gp/product/B01C3RFHDC/