Fixed in revisions 8253/8254. Instead of inserting into the model directly, we now just call setProperties just as in the no-append case of importing. The difference is that the properties being set are identical to the current properties, with the imported elements simply added to the list.
Also in setElements we now explicitly set the sequence numbers ahead of time. This is because tree-table nodes will actually check equality of nodes when inserting, so two identical elements would have been ignored.