Thanks for the feedback, appreciated. The reason it suggested that route was (1) the park, as you found, and (2) the Via Francigena hiking route (https://www.viefrancigene.org/en/ & https://www.openstreetmap.org/relation/6201614) follows this road.
One item for consideration in the future is allowing users to flag unsafe portions of a route and then having that visible to other users (and also influence the algorithm).