For a simple example, create a Main menu, then a Submenu with one child, and another Submenu with one child:

PROBLEM: Now switch from Design to Code view, then switch back from Code to Design view.
As simple as that it breaks:

and trying to unbreak them isn't quick or easy when you've got quite a few menus, and even if you can you can't edit any further in Design mode because it just keeps reintroducing the problem
Any version of PB on Windows, not sure if Linux/Mac are also affected as i do most form designing in Windows
