Export-import does not reproduce the original structure of items in collections (v.6.0.27)

I am exporting a collection with two hierarchical subcollections. There are two items in each collection.
ZZ: A B
--|
--YY: C D
----|
----XX: E F

Re-importing of the output file generates a corrupted structure of the distribution of items in the collections compared to the original one.
ZZ: A B C D E F
--|
--YY: C D
----|
----XX: E F
In the root collection we end up with six items instead of two.
Is this the way export-import operations are designed, or is it a bug? (I work with Zotero v.6.0.27).
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