andori, apologies. I did look at this when you first posted, here's what I found....
When we stack things in MC, MC assigns the stack a (seemingly) random stack ID of around eight numbers.
I expanded a stack, then exported the mpl list and removed the files and stack data from the library. Well, I checked the "also remove all stack files" check box on the delete dialogue, even though I had all of the stack files selected for deletion anyway, as I believe that that would also remove all of the stack data.
I then re-imported said files. There were 21 of them.
They did not stack.
However, when I rebuilt the stack by way of selecting the 21 files, and right clicking on my 'top of stack' file and using the stack menu, MC placed them back into the same stack ID as they had been before being deleted.
As I couldn't see anyway to have files imported to the stacks database in this way, I shrugged my shoulders and moved along, conveniently forgetting to send you a reply in the process
Now, while I have been typing this reply, a thought has occurred...
I wonder if the "error check and correct stacks" option would have fixed that stack?
Also, if it interests you, I wrote a kind of 'how stacks work' thing a wee while back...
http://yabb.jriver.com/interact/index.php?topic=55782.msg379242#msg379242