My iPhone 6plus refurbished( 2nd replacement phone ) decided to Automatically HARD RESET today while being charged. It wiped out 1500 pictures and videos, erased all my passwords and deleted a few APPS, lost contacts and text messages for work. When the screen showing the APPLE finally went away and it started back up, I had to go back through all my apps and log back into each one, resign up for my email, and icloud. Luckily I backed up my iPhone 3 days earlier. It took 3/4's of my day to do so, but I'm still logging into more apps. I took it to a Verizon store in Alisa Viejo, CA and they said its an APPLE issue not Verizon... WHAT A JOKE....I'm not going to run all over town back and forth to figure this out, because you know Apple will say its a Verizon issue not theirs.. Needless to say this refurbished iPhone takes 15-20 mins longer to charge than my friends brand new iPhone 6plus S... Good thing I only got 6 months left on my contract... hopefully by then the iPhone 7plus will be out
Never heard of an iPhone hard resetting on its own and deleting things. But if that happens I doubt it's because it's a refurbished device. I would imagine something is specifically wrong with it. I've had refurbished iPhones before (well my children have) and they all have worked perfectly. One, an iPhone 4 still works although it's slow by today's standards of course lol. Also, trying to compare your iPhone 6 to a friends 6s which is a newer model isn't the most accurate way to prove there is an issue with your device.
If you have an Apple Store in your area and you are still under warranty then take it to them. I wonder if maybe you have some damage though...such as water damage that may be causing this to happen. If that is the case (which the Apple Store would be able to verify and show you if that is the case) then you won't be covered unless you have AppleCare+ or some other insurance.