An empty folder shows an index card in it, and a folder with sub documents appears empty. See Screenshot (notice the arrow by Gettysburg indicating more documents):
The folder with an index card on it has text in the highest level, the folder itself.
At least that’s true on the Mac side.
Yes, the key thing to grasp here is that folders are like files. They can have synopsis and even text content like a normal file. The icons on the folder indicate whether or not it has content. In this case, the folder has some text in the synopsis field (what shows up as an index card in corkboard), but no text. If it had text, it would have a small page in the corner. The icons mean nothing about the files beneath the folder.
It still feels misleading to me. Both of those folders had text in the synopsis, but only one had an index card, and it disappeared when I added a file to the folder. So, from my perspective, the index card was an indication of an empty folder, and an empty folder icon was an indication of one with subdocuments. When I (just now) created a blank document and made it a folder, then I saw it as an empty icon–like you said I should. And the index card appeared when I wrote some text in the Synopsis.
So I guess the question now is whether or not the index card is supposed to stick around after documents are added?
Ah ha, I see what you are getting at now. After playing around with it a bit further I do agree this is a bug. The folder badges should stick no matter what their content is.