Q: What's the meaning of 'Filename (Original) 0' and when does it disappear?

A: The filename 0 is a XML tag of the BCS. Each element scheduled in BCS has a filename 0 which points to the original sound file.
There is one exception: Files on the local hard disk of the scheduling client which will be put into DigAIRange do not get this metadata because DigAIRange copies the file automatically to the primary media directory. Therefore you have to set the corresponding flag under server settings.

This is necessary because a central BUS cannot copy files from other computers pointing to a local hard disk like D:\sound...

In such case you don't have such a tag for the element. It would start with 'Filename 1', generated by DigAIRange and pointing to the primary media directory.