Yeah, I have seen a problem where boolean union can in certain situations fail to glue together all the edges of adjacent surfaces. Once you have some edges that are not glued together it will stop your model from having a true volume and it will behave like an open surface.

Like Steve mentions using Edit/Separate and then Edit/Join to repair this problem for now.

I am going to be adding in a kind of "edge gluing" pass to the boolean process to fix this up, I'll try to add that in for the next v2 beta release.

- Michael