It’s possible because of the way LightBurn stores paths internally to have data stored in a form that makes it hard for the existing code to join it. Break Apart followed by Join basically rebuilds the shape to follow the ‘proper rules’, which is why it works.
One of our longer term plans is to rewrite the under-the-hood way we store stuff so this can’t happen, but that’s a huge undertaking that will touch a lot of existing code and break a lot of stuff.