I haven’t been able to stop the output coming through the mic input via pulse audio. I found a hardware hack, get a usb to 3.5mm headphone jack adapter.
No loopback present? Check with pactl list short modules
A quick online search for similar reported issue suggested “Check Discord input device” to make sure it’s set to your actual microphone, and not “Monitor of …”.
Ok, so not a software issue as such. Your sound card is mixing output audio into the rear mic port. It might be a combo jack mapping issue (not really sure) causing misdetection perhaps. Your use of a USB-to-3.5 mm adapter is probably the cleanest, pragmatic solution to avoid the hardware audio mix.