I rarely use my mic. However, when I first started playing on XI servers I had a shitty headset. Apparently the mic was not working. Someone told me to pull my mic our of my ass. I'm not sure if this helps or not, but I did however go buy a new mic.
But no, really, the only issues with discord ive had the few times ive used it.. was realizing I could turn down the music bot and still hear people in the raid talking. (For when I was playing WoW Classic.) Does this also happen when your using TeamSpeak, or only Discord?
Maybe try push to talk if your not already using it? Would at least minimize the back feed your streaming into the channel. As for the actual issue for the back feed issue, im not sure. If your not using push to talk and your putting your headset on the desk, I would assume the mic would be able to pick up on the volume of the headset and feed it back if it is loud enough.
All though, is if you recently purchased the headset, try and exchange it if possible. Would eliminate faulty wiring in the headset itself.