I notice Cycle.travel, along with OSRM and BRouter (and the OpenCycleMap map), ignores one-way streets with contraflow bicycle lanes/tracks that are tagged with either "cycleway:left=opposite_lane/track" or "cycleway:right=opposite_lane_/track". It only understands "cycleway=opposite_lane/track".
As a result, cyclists are made to take a long detour needlessly.
The cycleway:(left|right) key is especially easy to add since this is the tags Id uses by default when editing OSM through a browser (you have to manually edit it to remove the ":left|right" part). But then, contraflow lanes/tracks are pretty rare in OSM, possibly reflecting their rarity on the ground:
Here's an example in Cycle.travel and OSRM (avenue Augustin Dumont is tagged with "cycleway:left=opposite_lane"):
This seems to me like a serious issue.