I may have to reach out to ODOT and see what they have to say about where OR 39 and CA 139 terminate.
Real quickly, I threw together a mockup route with two points, one at the CA 139/161/OR 39 point as marked in OSM, and one at 42 North.
The junction, and the OR/CA border, is at 41.998268 North, and 42.000000 North is .12 miles north of that... which, if that is the correct location of the border, places CA 161 firmly in California.
On the other hand, CA 161 is on State Line Road, and the CA/OR (and NV/OR) border is not a straight affair in the best case to begin with...