I went digging through the Apple Support forums, and found a few topics about this. Most people believe that, when your iPhone is plugged into your car to utilize CarPlay, that CarPlay pulls certain information from your car's GPS or head unit to be more specific regarding your speed and location. That would lead credence to my observation that this nonsense began after updating my car's infotainment system to the latest software.
Although, one person had this to say:
Hi, and thank you both for your replies. However, both I and Apple disagree. Yes, I read the whole thread, so I saw the replies where it was suggested that the car manufacturer is at fault. However, if you read the entire thread, you will see people have this identical issue with Fords, Porches, Subarus, GMCs, and others. If it is an issue with Ford's system, it is apparently an issue with every car maker mentioned in this (and the other, numerous) threads. That alone suggests strongly the problem is Apple's, not the various car makers.
Second, yes, I and most in this thread know that CarPlay derives its GPS from the car when on USB, not the phone. So, it is mysterious that, as I specifically wrote (and have others) the GPS in the vehicle is completely accurate using its own navigation screen. It is only when interfacing with CarPlay that things run amok. This factor alone is not indicative of whose system is at fault in corrupting the transfer of data from the car's GPS to the iPhone, but it shows that CarPlay is the conduit for the fault.
Third, posts in this and the other numerous threads, in this and other forums, suggest the problem is intermittent. Mine is no different. I took three car trips yesterday around my city. I posted after the first. The second trip yielded perfect results on CarPlay--exact/correct locations found on each map app. The third trip had the issue again. My one trip so far this morning had no issue at all.
Finally, and maybe what I should have led with, yesterday evening, I went to the Genius Bar at my Apple Store and asked about the issue. The Apple Store employee stated, unequivocally, that this is a known issue with CarPlay, and is being worked on at Apple.
I probably should have just gone straight to the Apple Store before posting here, but, I'd just done a Google search, and thought I'd add my similar experience. I've owned Apple products for 34 years. I find it frustrating when they don't acknowledge a problem that seems widely reported on Internet forums, but, since the Apple employee last night fully acknowledged that this was an Apple problem, and that they are working on it, that's good enough for me; I wanted to pass that on to others in this thread who have been having this problem crop up. Thanks.
So, I dunno right now. I'm going to look into doing a hard reboot of my car's systems, and see if that helps. It's possible they only go into a "sleep mode" when the car is off.
[doublepost=1566528081][/doublepost]Someone said this, back on April 30:
Contact Ford. The head unit needs to be updated. Changes were applied recently to the Global GPS system to account for drift of the magnetic pole. The nav unit in the car is probably using an old database that does not account for the change. When you plug the phone into the head unit, it gets GPS data from the head unit instead of the phones own built in GPS.
That's interesting. I *did* notice that the compass on my car's rear view mirror seems to be a bit off. Like when I'm heading east, my mirror says I'm going southeast. They rarely match up.