It seems that no matter the circumstances, on a modified build of Party Project from the latest version of workshop, the shadow star space always takes away a star. This affects vanilla Party Project boards as well like Mario's Rainbow Castle.

I found out more about this situation. It turns out this behavior occurs when a custom board has the "Shadow space takes away stars" option enabled. If one custom board has this option enabled when added to a build, it affects all boards, including vanilla boards.