So the Test isn't Failing
It turns out, the test I prepared earlier passes. So the reported issue needs a bit more work. I wanted to avoid the data-set which reproduces the failure because I'm lazy.
I should have known better.
So I will now do a similar manual step through the data-set as I did earlier... This time, as the data is one-dimensional, I will need to find anotehr way to visualise it. And I also think I will do that by manually writing an SVG file. Clicking all that together with a mouse would be painful. Let's see where this gets me...