I also work for a company that makes a GPS fitness app and talked to our iPhone guys about this.

They said it does snap to roads when you’re going over about 15kph and there is no way to know when it is doing it, so you can’t mark them or anything like that. If you’re traveling around the threshold you could have weird sections where you snap for a bit and then don’t for a bit.

They did say you can turn this off by NOT setting your app as Fitness. Yes this is backwards from what you’d expect, but that is what they told me.