No, it's a sign that the battery was sitting around for a while. Ask them to make a record of the fact, since determining whether there's a problem will take longer than the 2 weeks they normally give you for an exchange for a new phone.
Fully charge it. Use it until the phone tells you to recharge it. Do that 3 times.
Now use it normally and see if you get what's normal for that carrier's S6 for your area. (A weak cell signal gives you less battery life, running more apps at one time does too. So does being on the phone all day.)
If the phone works normally, the battery will probably hold up for as long as it should. (Never let any lithium battery get below 40% charge unless your life depends on it. Use a dead battery to call 911, but not to tell your wife that you'll be getting home late.)
If you find that you're getting a short time between charges, there's the record that you reported the problem while the phone was still under an exchange warranty, so ask them to exchange it. (That's one reason I don't like phones with non-removable batteries. A $15 problem turns into a big deal.)
I got a phone in for repair recently that, if the graph went down that low, would have been at about -300% charge. I charged it, cycled it a few times, and it's still running as well as a new battery. One time deep discharge will shorten the battery life less that the difference between individual batteries. Constantly trying to get the last bit of charge out of it can kill the battery in a few months.
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