The relevant part of the PHB ranger's Natural Explorer feature states (emphasis mine):
While traveling for an hour or more in your favored terrain, you gain the following benefits:
- Difficult terrain doesn’t slow your group’s travel.
- Your group can’t become lost except by magical means.
The way I read this, per RAW, is as follows:
When a party with a ranger in his favored terrain starts travelling, the first hour of travel the feature is not active and will activate only starting at the 2nd hour of travel. So they could become lost during the first hour of travel if the navigator fails a Survival check. If they stop travelling for some reason and resume travel later, they then need to again make Survival checks for the first hour, and then Natural Explorer will kick in again at the 2nd hour.
Am I correctly understanding the Rules as Written for the Natural Explorer feature?
(NOTE: We are not playing with UA rules.)