OK it looks like there are two Borderlines - the more accurate one has been imported from here:

osm.wiki/Import/Catalogue/Kreisgrenzen_Deutschland_2005

while the "Austrian" one is not visible in the Editor and looks like it consists of 2 Layers, one darker
which correctly shows de-cz while the other a little bit blurred line shows the wrong country.
so it looks like its some sort of Orientation-Overlay which will be removed some time.
Another good Example is near Mähring,Oberpfalz on the Border crossing of The ST2173 -
here even the accurate line shows the Österreich-Overlay.