iPhone shuts off when battery life reaches ~40%

I replaced the battery on my IPhone 6 a couple of weeks ago with a battery in your replacement kit. Now just a few days ago, my phone will shut off when the battery life gets to around 40%. Will not let me restart the phone unless I connect to a charger. Initially when I connect the charger, the screen will show a dead battery, but then shortly thereafter, will jump up to the % life it was before the random shutoff. I followed the battery calibration instructions upon initial install. Not sure if this is a bad battery out of the box. Please advise. Thanks.