Some 3.5 plugs have a steel pin for the central pin to make them stronger. So the left channel is going through steel rather than brass or some other non ferrous metal. Most TRRS I have run across have this because of the smaller amount of metal or thinner due to four conduction points in the same 3.5 plug vs. the TRS with just three. Also they often use something in the plating that can be magnetic, even in the real expensive stuff, so I always check with a magnet.
I ordered a dhc fusion iem cable terminated with balanced trrs from peter. Should I contact him to make sure the trrs he is using to make the cable isnt magnetic? I can probably trust a cable maker though?