Ok, I just merged a fix for this case, which you should be able to get with the LB Versioner in a hour or so:
From what I can tell, the change that McNeel made was completely under the hood of the
Rhino.Geometry.Brep.CreateContourCurves function and nothing publicly changed on the method itself. But it’s clear to me that certain intersections towards the bottom of the Brep that were succeeding previously were no longer succeeding in Rhino version 7.15. So my workaround is just to add 2 * ModelTolerance to the lowest section plane used to make contour curves over the Brep and this seems to get it to work. Importantly, I needed to only add this 2 * ModelTolerance in Rhino version 7.15 and above since this addition seems to break things in the earlier versions of Rhino.
If @piac or @wim has a suggestion for an alternative fix for this case that doesn’t involve me sensing the service release of Rhino, I would be interested in hearing it.