I'm trying to get a pre-fader mix sent from my mixer (Alto ZMX 862)

https://www.altoproaudio.com/images/..._ORTHO_WEB.jpg

The Aux 1 (Mon) is pre-fader.

But whenever I plug anything in to the Aux 1 Mon output, all I get is one sided audio.

Does anyone know what these Aux sends are?

Guide says:

You can use a 1/4" TRS cable to connect the AUX 1 MON output to the
input of an external amplifier or active monitor to create a custom monitor mix for onstage musicians.
You can adjust this level with the AUX RETURN LEVEL knob labeled "TO AUX 1 MON." To use an
external effects rack unit, compressor, etc. with the mixer, you can use a Y-cable to connect the AUX 2
FX output to the input of your external device, then connect the outputs of the device to the AUX
RETURN INPUTS of the mixer.

Does that mean it's a mono single channel output?