Something has gone wrong in the last update, take a look here http://cycle.travel/map/journey/24044
South of marker 1 the Monmouthshire & Brecon Canal is marked as a road, to the North the road zips off in a straight line and then follows/becomes the Nant Lago for a while. There doesn't seem to be a single feature in OSM that would explain it, the boundary of the Brecon Beacons National Park is closest. At least, there isn't a single feature today but could have been when the update ran.
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Ah yes – I spotted that the other day! A weird edit in OpenStreetMap to the BBNP boundary that triggered an edge case in osm2pgsql (the database loading software). I’ve manually removed it from the cycle.travel rendering database, and had a word with the OSM mapper who’s now deleted it in OSM (http://www.openstreetmap.org/changeset/39164389). The routing uses a different loader so shouldn’t be affected.