In this case, it looks like there might be a restriction for pedestrians on that section of the road, because Mapy.com allows it by bike, as shown in image.
Mapy.com doesn’t allow route planning where it conflicts with access restrictions - for example, cars can’t go into forests.

Such issues do occur occasionally, but they’re rare and mostly due to inaccurate data in the background maps, so Mapy.com doesn’t usually deal with them directly.
You can probably fix it yourself by logging into OSM maps and reviewing/editing the data for that road segment.