Thanks to the several people who pointed out that the real problem is likely to be an error in the mapping of the coastline, unconnected with my edit and possibly a long way away. I added some footpaths to the map, and I then saw that, as the edited tiles were re-rendered, the land appeared coloured blue. I thereofre therefore assumed that my edit had caused the incorrect rendering, but it seems that I was wrong.

Thanks to the several people who pointed out that the real problem is likely to be an error in the mapping of the coastline, unconnected with my edit and possibly a long way away. I added some footpaths to the map, and I then saw that, as the edited tiles were re-rendered, the land appeared coloured blue. I thereofre assumed that my edit had caused the incorrect rendering, but it seems that I was wrong.