Looking at the border north of Moldava, the German border includes many segments of less than 10m that follow a very irregular pattern on flat land, sometimes turning back on themselves. Do these reflect historical landholdings? It’s a surveyor’s nightmare.
The border you are referring to (dividing Czech Moldava and German Hirschbach), actually does not go through "flat land", but the surveyor's nightmare part copies the brook that runs through that part of the valley. You can easily see it even on Google Maps' satellite images, where a darker trough/bed is visible pretty much copying the border (with some minor deviations here and there from natural erosion and other such things that changed its shape over the centuries).
Or, to be more accurate, it is an entire network of brooks and creeks that make up a lot of the border. Along Moldava, it's dominantly the Moldava brook (Moldavský potok) and Polterbach.
As you head east along the border, the brook eventually dries up and disappears, which is why the eastern part of the Moldava microregion has a lot more reasonable bordering that presumably does not lead to damage to an average cartographer's sanity :)