S6+ Deep Sleep

[Cop Tran]

I just got my S6+ yesterday and did all the setup. The phone went into a deep sleep last night when the battery charging indication "on" all night and I was left with 40% of the battery left in the morning. The phone will not come out of deep sleep mode unless I simultaneously "hold down the power and volume buttons". It intermittently does this. Has anyone experienced this problem? I have called ATT and they asked me to send the phone back and they will replace with a new one.