Basically, if you use any info other than just the postcode, it ignores the postcode. If you add a road to the postcode, or then chose the final location from the list, even if it offers only one choice, it will use the road name and not the postcode. If there are two roads with same name in the vicinity (which could be several miles away [or more]), there is a good chance that it will set navigation to the wrong location as it is only using the road name. Particularly bad when roads are split.
Try setting navigation to 'London Road, RG40 1SS' which would not be unreasonable if you did not know the destination. It will navigate you to the wrong side of the A329. RG40 1SS actually a school.
To compound matters, if you use an app to find a location and say drop a pin or look up destination details, then share that to the car (ie iOS / send to Tesla) then it would appear to send a location string, including road name. So even though you have used an exact map location, Tesla sat nav will reinterpret that and again, if there are roads of same name in the vicinity, it may end up choosing the wrong one.
In the UK, the postcode should be the definitive location if supplied, but the road name seems to take priority.
In this post. I dropped an exact pin location. Tesla sat nav took me to Church Road Horsell and not Church Road Woking (Horsell is in the borough of Woking). It took a 10 minute detour and faffing around when parked up to actually find the correct destination.
Google Maps - send to car