They are all “Closed Breps” so doesn’t that mean all their face normals are outward?
To compound my frustration, I created a new, empty Rhino file to prepare the posts above, then switched back to my original GH file which was getting its geometry from Data Input, so I didn’t notice that I hadn’t reopened my original Rhino file. Much to my horror, a couple of the sliced breps now failed to Cap Holes, though I had made no changes to affect that. Took a few minutes before I re-opened my original Rhino file and all was well again.
Apparently SplitMul (Split Brep Multiple) and/or Cap Holes (not sure which) was happier with ‘Units | Absolute tolerance’ set to 0.001 (the default for a new ‘Small Objects - Feet.3dm’ file) instead of 0.01 (the default for a new ‘Large Objects - Feet.3dm’ file).
I’ve said it before and will say it again; Rhino is the most aggravating software tool I’ve ever used. Coding has always been a refuge for me and when Grasshopper works as expected, I still get that satisfaction from it, but unpleasant surprises happen way too frequently. It’s not safe.