An XBRL instance may validly contain a nil-valued fact, and have one or more footnotes attached to that fact. There are many cases where this is needed.
When such an instance is Imported during the 'save as xbrl' function, the nil valued facts and therefore their footnotes are not imported.
Logged In: YES
user_id=896286
Originator: YES
Actually, it seems more severe than that; footnotes are always dropped on import no matter what they are attached to.
Thank you for submitting this defect. I have forwarded it to our QA Department for review.