Weird earphone problem with iPhone 6s

Son just received his iPhone 6s today. Bought refurbished - phone is absolutely mint and works perfectly. Unused, sealed pair of earbuds/earphones in box - tried these, and although they sound fine, the in-line controls don't work - but only with the iPhone 6s. Tried them on another iPhone (5s) and an iPod Touch and the controls do work. Problem with the socket on the iPhone 6s? You'd have thought so, but then tried another set of earphones (JVC) and some headphones (Musical Fidelity), both with in-line controls, with the 6s and they both worked perfectly.

So we have a set of Apple earbuds with inline controls that work on other i-devices, but not on the iPhone 6s, and other sets of headphones that work across all the devices. I'm baffled, tbh. Obviously not a dealbreaker - just get some other earbuds, but I'm curious as to where the fault lies.