[bug report] what does the depth of El tree node data response have to do with?

### Element UI version
2.15.1

### OS/Browsers version
mac

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/5pjnqs4u/5/

### Steps to reproduce
```

export default {
data() {
return {
id: 0,
// data: this.$store.state.sidebar.menu
data: [
{
id: 1,
label: 'nodea',
children: [
{
id: 3,
label: 'nodea3',
children: [{
id: 5,
label: 'nodea5',
}],
}, {
id: 4,
label: 'nodea4',
children: [],
}
],
}, {
id: 2,
label: 'nodeb',
children: {}
}
]
}
},
mounted() {
this.data.push({
id: 117,
label: 'oooo',
children: [],
})
// this.data.splice(1, 1)
console.log('children:', this.data[0].children[0].children)
this.data[0].children[0].children.push({
id: 817,
label: 'xxxx',
children: [],
})
// console.log("data,", this.data)
},
methods: {
add() {
this.data[0].children[0].children.push({
id: 1017,
label: 'addxxxx',
children: [],
})
},
}
};

```

### What is Expected?
1. 页面初次载入后,应该可以看到节点xxxx,但是并没有.
节点ooo可以看到.

### What is actually happening?
1. 没有看到节点xxxx,但是看到了节点oooo
2. 点击testbutton,然后看到了节点xxxx和addxxxx