I think it is battery issue. Here's why. I just bought new original wall charger (before I charged my phone thru PC(usb) and my phone is charging +10% every 7min, that's def. not normal
I remember that old Nokia phones did that simptoms when had old/bad battery, but I don't know how it is with these Li-on battery. I have Makita power drill with Li-on battery and it takes less and less time to fully charge every year as battery gets older, so maybe these new type of batteries are the same.
I'll definitely let you all know if replacing battery helped.
I don't doubt that you have a battery issue but it isn't crazy that your battery is recharging that quickly. My 6s Plus just went up 9% in 11 minutes and I was using it.