Hi Linus,
For my experience with Samsung phones, happens the same as you, they heat without any reason, or so I thought...
Turns out that now since I'm stuck with S7 (not edge) it would only heat if I ever keep a game in standby on the background, with wifi or 4G on... if I keep all apps closed, it would not drain as fast. Hope my comment helps you to determine the problem and come up with a permanent solution.