Something seems to be shorting out. The 6S is an old device and after years of pulling it in and out of your pocket, it’s likely collected some gunk in the port.
Make sure the port is clean. This isn’t super easy but some compressed air shots can loosen stuff up. If you get in there with a toothpick or something, be very careful, once something breaks, it’s done.
once clean, as others have mentioned, use quality earbuds. Plenty of companies still sell the 3.5mm plug style earbuds.