[bug report] [bug] the echo data in cascade component

### Element UI version
0.1.0

### OS/Browsers version
Windows10/Chrome 80.0.3987.132

### Vue version
3.0.0-alpha.9

### Reproduction Link
https://codepen.io/tianzhihen121/pen/MWwXwOK

### Steps to reproduce
Delete first tag，you will see the first tag is still exist，but the third tag is deleted.Wow!!!

I think the reason is related to cascader component defualt sortings rules，
ex:[1,2,3][1,2,4][1,2,5] order is right，but [1,2,5][1,2,4][1,2,3]，when you delete[1,2,5]，it will be wrong

Please check out,thank you!

### What is Expected?
Delete first tag,first tag will be clear, And third tag still exist.

### What is actually happening?
Delete first tag，first tag is still exist，But third tag is deleted