I got my 6S on launch day. Sometimes randomly, the phone would die when the battery is between 25 and 40%. I don't know why this happens, and I tried restoring it already. I went to the Apple Store and they told me again to restore it with iTunes and set up as new. I tried that, and this issue still goes on. This has been happening for about 3 months. Do I need a new phone or could be a software bug?