Using those postcodes, a Garmin device using the current cartography (provided by Here, as suggested by
@steve_w) routes north on the B1347, following the route shown on your screen grab of Google Maps.
If I use the route planner at
HERE WeGo then it offers the longer route, first heading west along the B1377 before heading north on the B1345, as the recommendation for car travel, but the shorter route north on the B1347 for cyclists. Interestingly, if I experiment with the routing parameters that Garmin offer and ask it to avoid Residential Roads, then it follows the longer B1377 / B1345 route favoured by the Here routing and, presumably, MBUX.
Based upon this, my guess is that Mercedes have chosen to favour avoidance of Residential Roads in their routing algorithm - which makes a good deal of sense, even if it may result in what looks like a perverse result when asking for a shortest route connected by residential roads.
So regarding your question "should I just accept that it is unnecessarily going to add one third to my mileage every time I use it?", the answer is "No", except in very special circumstances.