There are entities/(invisible) materials to prevent access from both sides; it isn't broke; plus there are not really two cubes (one is what is seen through the portal). Hammer has more ability than the basic Portal II editor for configuring pass restrictions (blocking/passing of objects) with key settings tied to object types, names, classes. And it may well be the grate default was set to be a one way block, not both ways; so, in which case what the video shows is correct behavior.

I don't believe I ever said there were multiple cubes in this video, but alright. I'd rather not have to learn Hammer just to prevent this simple problem from happening. I've never seen grates behave like this in the game before. If they can't block objects, they're pretty pointless.

I was sharing my observation that there can seem like more than one cube, is all. Yes, the built-in editor is extremely limited; you can't even do sloped passageways and similar. Hammer is ancient, but still a good tool.

This is one of Portal's many glitches. See my guide
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=163045846
and particularly the page "Portals (3) - Through walls"