This has happened to me on occasion for at least a year now. I never raised it since I suspected I should do some more investigating on it my own behalf first.
It could be my own setup, since it isn't something complained about very often.
It happened again recently, and doing a forum search on it I thought to piggy-bag this thread.
When this is happening, and I keep an eye on the contents of the newly created library folder, there is no occurrence of the new mbl file appearing in that folder at all.
It only contains:
Should the mbl file be created shortly after scanning is completed, or only at closing MB?