I can’t answer the question but maybe I can clarify what is being asked about as I have also noticed this. Some properties have a note saying “Distance in property description is calculated using © OpenStreetMap”. If anyone wants examples, searching for that exact text and booking.com should turn up plenty from a variety of countries. This appears to be completely independent from the map display (which is Google Maps in any example I have seen, even where that text reference to OpenStreetMap is included).