iPhone 6 Plus sporadically turns off <60% charge, bad battery?

Good afternoon,

I have purchased an iPhone 6 plus on eBay from a reputable seller. The phone was not 100% new but I was advised it was a stock model and in good condition. When I received the phone (without retail packaging) I had no indication to think otherwise, it did not even have a scratch on it.

I've had the phone for about 2 months and a strange issue began to surface. For some reason, when the phone's battery is below 60% and I use it outside to take a picture or watch a youtube video, it completely turns off. The phone will not turn back on unless I plug it into a wall charger. It does not have to plugged in for long, even for ~5 seconds is enough. It's almost like a car with a dead battery that needs another car to help it jump-start.

When the phone turns back on it shows the same battery level as when it died, i.e. is not at 5%. I should also add that this does not happen every single time, but I would say it happens over 50% of the time, which is obviously too much too tolerate.

I read through similar iFixit forum posts about battery issues, so here's my question: does it sound like an old battery issue or a damaged tristar ic (or something else?). The phone came with a brand new and, from what I can tell, genuine Apple charger, although I cannot vouch if a non-apple charger has been used with this phone in the past.