My wife has this iso changing issue on her M6 and so do I when I use it. What happens is that when you turn the camera off, you turn the dial with the on/off switch. So you need to be careful to only move the on/off switch. It's very easy to move the dial this way.
Thanks for your response, but this isn't my issue. The dial is detented and it takes quite a lot of effort to move to each click. If I accidentally turned the dial while turning the camera off I would hear and/or feel the click. I don't. When I first started experiencing the problem, I thought exactly the same as you. In an attempt to overcome the problem I disabled the dial through the custom functions so that it didn't do anything when turned. However, the problem didn't go away.
I've just tried again turning the camera on and off multiple times and whenever I do so my finger doesn't touch the dial. I've always wanted to believe there was a simple answer, but I have never found one. Also, this wouldn't account for the fact that occasionally when the camera is on I look at the screen and see that the ISO scale has popped up for no reason. The camera seems to have a mind of its own.