Thanks for your reply. Probably the fact that my iPhone is disconnected from wifi is caused by my router, not by PE. Apparently when downloading map data, my iPhone (but not my iPad) does things that my router does not tolerate.
But I’m 100% sure that, even when PE is in WiFi only mode, my iPhone sometimes switches to cellular data, and in that case PE continues its downloads using the 3G network (which costs me a lot of money once my limit has been reached). When downloading maps to my iPhone, the only thing I can do, even at home with my WiFi network up and running, is to disable Cellular Data completely.