Hi Martin, that's actually how boolean merge is meant to work when open surfaces are involved. When there are open surfaces it will try to extract all closed volumes it can and remove the exterior surfaces, there's another example here: http://moi3d.com/3.0/docs/moi_command_reference7.htm#booleanmerge , see the one labeled "Boolean Merge can also be used to extract a common solid volume from a set of intersecting surfaces:".

> If not a bug, then why does that particular part go missing?

It's because it's an open surface and not a closed volume after everything has been intersected and merged.

- Michael