No fix because it's not a broken feature, it's a safety feature. Most phone manufacturers are applying that automatic volume re-leveling when plugging in headphones to avoid loud, sudden noises blasting into your ears, preventing damage/hearing loss.
Samsung phones, for instance, not only do that, but they also add a very annoying (but very valid) warning pop-up when you try to go over a certain volume level while using headphones. It pops up once per reboot and there's no way to disable, dismiss permanently, or ignore it.
10-17-2016 10:31 PM