The key issue seems to be - as Helium314 said- that the blue pedestrian area is not a simple area - but a Multipolygon. None of the 3 OSM-routers is able to understand this.
Solution: We could re-build the pedestrian area with simple areas. And/Or we could add a "footway"-way crossing the pedestrian area. Advantage of 2nd solution: Routers are directly routing along the footway and not the edge of pedestrian area.