This ship is going from Falgwith to Ponzeath, with no intermediate waypoints. It sails to the position indicated in the screenshot, then turns towards Ponzeath. I've noticed that behaviour before in the last game.
Thank you for the report. Are you able to upload a saved game in which this can reliably be reproduced?
Not a bug. That's what my "backup ship pathfinder" does if the distance is too long to use the conventional routefinder. It follows a straight-then-diagonal route but detours around land. I could maybe make it a little smarter at figuring out which of two possible such routes to use, but in this case, there is no straight-then-diagonal all-water route.
We needed *a* routefinder for long distances because it has to be used to test things like "Is the fishery reachable by sea from the fishing port?"
But I don't think it's worthwhile to put a lot of effort into making it good, since the problem can be fixed by adding a waypoint.
Taking non-optimal coast-hugging routes reflects historical travel patterns with ships anyway :-)