I've noticed on the HUD if you have the line speed indicator bubbles on it reads in the wrong units if you have it set to automatic units. I will include a picture when I am able as I am currently at work.
In this picture you can see in the top right the next indicator is in KMH the speedometer in the bottom right is in KMH yet the bubble on the line is in MPH.
Ive done some further poking around and I noticed that if I went to a route where the units are in MPH it was fine. I tried multiple KMH based routes with the same result (speed not matching in terms of units). I then went and changed the units from auto to Kilometers, then back to auto. It fixed that instance however now I loaded into Sand Patch and now the speed change is showing in KMH (in that bubble) for the next speed zone. It seems like if you make a change to whatever the native speed measurement is and then go back to auto it keeps in instead of auto detecting as it should.
I have noticed that the power/braking force units on the main HUD don’t change automatically sometimes, when switching from a metric route to an imperial route when you have automatic selected. That’s on PS4 so the automatic switching not always working fully is probably a global issue. It’s not become a persistent issue though so have never reported it before.
So the answer is no it does not reset to native region speed. I also tried full HUD off and on also didn't reset to local units.