Ran into this with the iPhone 7 - IOS 10.3 and earlier.
When charging, navigation goes amoke. I first noticed with Garmin Street Pilot, which I use frequently. Keep in mind that this only happens while the device is charging from an external source. (I use a battery case which has it charging all the time and does not cause this)
I only recently tried it (wife driving) with the iFly app and had the same issues.
The result is that the speed is completly off, -02 Kts in iFly's case, and the direction (Heading) can be up to 90° off. The indicated position is off as well.
Unplug the charger and it pops back to normal. Plug it back in and it goes back to weird in 10 seconds or so.
I have worked with Garmin on this, tried different cables, chargers, with and without the battery case, cars, etc. Even exchanged my phone for a new one! Issue does not go away.
I have never used a charger in flight as the battery case keeps it running. The car chargers plug into the 12V auxiliary receptacle and I have used different ones.
The problem is subtle. It does not happen immediatly on plugin but at some point after that. Long enough after that I have not noticed it for as much as 10 or 15 minutes later.
This had never happened in my iPhones, 4, 5 & 6! I do not belive it has anything to do with the Apps, just something in the iPhone 7.
I am posting this as a warning and I am curous to see if any others have noticed such behavior?
Thanks, Jim