Phone exhibiting odd power drain issue. Too many cycles?

A friend of mine upgraded his phone and is giving his old iPhone 6 to his kid, so he asked me to clean it up and check it out. I replaced the battery in this phone with a quality aftermarket battery a bit less than a year ago. CoconutBattery reads the battery at 89.5% of its design capacity (1620/1810 mAh) with 666 cycles (yeah, really) which I think is a bit high for a <1 year old battery.

Phone came to me about 65% charged so I connected it to the charger. It fully charged to 100% and the back of the phone was very warm (slightly uncomfortable to hold for an extended period of time). I performed a factory restore on it and let it sit (unplugged). About 17 hours later the battery was still at 100%. Eight hours after that (25 hours after being fully charged) it was at 99% so I figured the battery was good.

39 hours later (64 hours after being fully charged) I found the phone *completely* dead/discharged. Remember, the phone was doing nothing; it was still at the “Hello” screen after the factory restore and was untouched the entire time. I thought it very odd that it would drop 1% in 25 hours but then completely discharge 39 hours later.

Right now I’m thinking it’s just a worn-out battery, but CoconutBattery is reporting it at 89.5% capacity which would indicate it still has life in it. I’ve seen phones with less than 80% capacity sit idle for longer than 39 hours. I’ve put in a new battery which is now charging; it’s drawing 1.2-1.3 amps and the phone is not warm at all. The old battery is not swollen or otherwise visibly damaged.

Am I on the right track here, thinking it’s just a worn battery?