I've had the same problem just now.
Before I plugged it in to charge my Z6, I had 15% on my battery. While I was charging it, it decreased to 7%. But when I checked the Battery Stats and the icon, it said that my phone was charging. Even the orange Notification Light was on.
What I did was I turned my phone on and off. And when it finally turned on, the charging thing went back to normal.
But I wonder, is this a hardware thing or is it just because I use my phone while charging it?