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I have not seen that ever with my iPhone.

If it were happening to me, I would try to use it when it did this "super-drain" thing to see if the phone truly lacked juice or if it was just a display issue.

In either case, if it were happening to my phone, I'd be on the phone with Apple arranging a replacement.

Let them figure it out.
 
You should probably go to the Apple store and have them check it out its a very uncommon issue.
 
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