*** Edited 11/14/2003 @ 7:01pm EST – see [edited] ***
Ok, this one is a little weird, but repeatable which makes my day. I am guessing there is a global property that is scoped to a FLA file. This property is the latest URL you have typed for a Runtime Shared Library Symbol. So, if you right click on a Symbol in the library, and press the Export for Runtime Sharing checkbox, the URL field will become editable. You have to put something in this field before closing the dialogue box. What you enter in there I think then sets this global property. If you make any new symbols and set their Runtime Shared Library checkbox as well, their URL field will display what you last typed, even on creation of said symbol.
I think there is another one for Import for Runtime Sharing. You can set it and the others follow suit, just like Export.
You can set them both via JSAPI & MMExecute like so:
Export