boolean difference a cylinder from a cube and it fails, if the cylinder is grouped to a non closed shape !
if its grouped to a simple cube sitting above the item to be ‘differenced’ it works.
Why should a grouping of solid and non solid fail when the non solid is not part of the required cut ?
(ignore the crude incorrect fillet, just a visual effect)
grouping upsets boolean difference.3dm (203.8 KB)