If your car was pointing the opposite direction to which Maps was expecting you to start moving it may have instantly rerouted without you noticing because you were heading east to towards Silverado and it may have calculated it was quicker to go that way, rather than attempting a U turn or to go round some blocks.
I have NEVER known it to change a route on a selected route unless I’ve a) ignored the route (either intentionally or accidentally) and gone a different way myself or b) it’s calculated a quicker route which it will do automatically, but it gives you a few seconds to hit ‘ignore’.
Re your adhoc points issue I’m not sure exactly what you’re saying. If you drop a pin anywhere on a map Maps always give you a directions button even if it’s not near a road.
Personally I always prefer this method as while Apple Maps routing is generally excellent I still don’t trust the pins and postcodes are too broad in rural locations in the UK where I live. I don’t trust Google’s pins either after a bitter experience 10 years ago trying to find an IKEA!
Couple of clarifications….
The reroute was via 12ProMax, not car.
In Google Maps I can have My Location as a start point and select any point on the may as Destination. Or the reverse.
In Apple Maps if I start with My Location, I can only select known points on the map as My Destination. No ad hoc points. However I can select an ad hoc point, drop a pin and then select directions to it.
Hope this clarifies what I was saying. Thx!