iPhone 6 Battery Draining and Heating only when listening to music

So, the iPhone 6 that I have does not drain battery on standby. Does not abnormally drain when I am using it normally. It gives me full standard usage as would any iPhone 6.

However, when I play music OFFLINE, it starts draining battery real quick. 2 hours of off screen playback cut down around 25 percent of my battery which is really abnormal. Phone also heats up a little bit.

The battery is at 91% capacity, and the U2 IC chip on this iPhone was replaced an year ago. Will any iPhone hardware experts be able to give me advice?

Before you go on about iTunes restore, my phone was in 9.3.3 jailbroken when this problem suddenly started to occur. I clean restored to 10.3.3, did not restore any backups. I have carried out these tests while on airplane mode as well. The point of heating is near the top portion of the apple logo.

P.S. : If you did not know, if your iPhone 6 drains battery on standby then it is 80% more likely to be a faulty U2 IC chip. My battery no longer drains on standby or any activity except for music through ANY app, though I only use the stock music app for this purpose.