When using the addChildren method to iteratively build a tree, lose the grandchild node

[Gryen7]

Version

1.3.0

Environment

Chrome 68

Reproduction link

https://stackblitz.com/edit/ng-zorro-antd-start-dz5eoa?file=src%2Fapp%2Fapp.component.ts

Steps to reproduce

1. Create a new grandchild node.
2. Building a child node using the addChildren method.
3. Building a parent node using the addChildren method.

I think the problem is here:
7c6fd1e#diff-b56c39a1c31e9a14fabede06a1c9408aL98

What is expected?

Keep all node.

What is actually happening?

Lost grandchild node.

[lock]

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