This is expected behaviour. If you wish to charge while it is turned off (although there no reason to do so), attach to the charging circuit, then turn it off.
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Because you just plugged it in. Depending on your settings, the phone will turn off again within minutes.
No it won’t. iOS devices have always powered on when off and then connected to an external power source. They will not shut down again unless someone turns them off.
Similarly, if on, connected to external power and then shut down, they will remain off until turned back on by someone.
Question: I have an iphone6s and when i charge it, i turn it off. when i plug the charge cord in, the phone turns back on. Why does it do that?