I have found that when my headset mic is in use (like through Discord), if I were to play a YouTube video as an example, the audio from the video will also play though my headset mic. The issue persists even when I physically turn off the mic. I have found that the issue seems to be due to the fact that I use a splitter but I am unsure why. I am positive that the mic is not picking up the audio from my headset (the issue is present even when I lower output volume to 0).
That depends on how well that splitter is made. It could perhaps introduce crosstalk where the signal to the headset is coupled into the wires to the microphine. That can be either through capacitive coupling or inductive coupling between the wires.