Sorry I do not have the solution..
But It seems to be a bug.
My client who has a magento shop (Magento ver. 1.1.8) that i configured for him
encounter the same problem…
I take some time to explain my client how to add category and sub-category.
He didn’t even clicked by error on the delete button cause he know he have to confirm the delete.
He encounters this pb 3 time in this last two day.
alxways the same pb he create a category, save it.
then try to create x sub category and save all for for each sub category.
but it always come to a point where the parent categorie and all its children cat are deleted spontaneously.
A chance that products are not affected.
I try to reproduce this bug but I haven’t been able to do so.
It seems to be a magento bug....
Anyone of the magento team can take a look at it?
Sorry but I do not have much more elements to give for now.