Another thing to consider is that in order to accomodate the new force touch sensor / sturdier aluminium build, they might have had to pack stuff closer together on the PCB. The battery is smaller as well after all. Something's gotta give, maybe in this case EMI rejection.
Then again, it's strange that both the 6S and the 6S Plus seem to exhibit this issue.
Looking at the teardown is sort of interesting. https://www.ifixit.com/Teardown/iPhone+6s+Teardown/48170