When a travel plane crosses an edge close to an edge point , it is possible that the above and below planes crosses different edges. In the current implementation one of the crosses is missed because we only check edges that are crossed by the main plain and the within result is wrong.
One possible fix is to check always the intersection of planes and edges regardless if they are crossed by main plane. That fixed the above issue but shows other issues like travel planes crossing two edges when it should be only one due to the fuzziness at edge intersections.
Not sure of a fix so I add the test showing the issue.