‘What’ says it’s closed, valid, and has 20 segments, but the ‘Area’ command does not compute.
Interestingly, just by moving the curve, or creating a copy, it becomes valid! ‘What’ will then say it has 16 segments.
will solve your problem
just run it after _contour to get proper polylines
(seams like _contour is designed to output polyCurves, one segment per Surface - and does not focus on outputting something special if everything is planar. But of course it would be nice if the output is filtered for small zero-length line-segments, therefor you might call it bug - @pascal has to decide this)