Mobile signal can disappear long before the path does. If you use a mapping app for a walk, download the route area while you still have a connection — then check that the map actually opens offline.
Don't just save a pin at the start. Zoom out far enough to include the whole route, nearby paths and the place you plan to finish. Some apps save a route line without all the surrounding map detail, so check what yours has stored.
Before leaving, briefly switch your phone to aeroplane mode and open the map. If the detail is missing, fix it while you still have a connection.
Keep a suitable backup for navigation, and don't rely on a phone with a flat battery.
Field tip: A map isn't offline until you've tested it.