Can't decide if the car is at fault or some interference is playing a part - either way it's very random. 

 

Occasionally my sat nav decides not to play fair - I'll set off and the location flag will not be quite right, maybe a few hundred yards out but that will gradually get worse as I drive. Some times it can be 100 miles out.

 

It's very frustrating because the speed limit warning appears to take some of the wrong sat nav info and sets off the bongs - once the camera picks up a speed roundel it settles down for a while, however, when the phantom sat nav location passes into a lower limit the bong goes off again.

 

Often it will get its act together within 10 minutes or so, but it can take over an hour / 60 miles or so.

 

Now here's the strange thing, Apple Car Play such as Waze - Google maps etc are wrong too.

 

Anyone else get this?

I had the same problem, which was fixed by a software update applied by the dealer.  The reason your phone apps are affected too is that

CarPlay takes its GPS data from the car to preserve battery life.