[Bug Report] `append-to-body` cause unexpected element display behavior

### Element UI version
2.13.0

### OS/Browsers version
Win10 / Chrome latest

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/mirari/pen/Powdgxg

### Steps to reproduce
Once the dialog opened, the div '444' which should be always visible will be missing when toggle the div

### What is Expected?
The div `444` is always visible

### What is actually happening?
The div '444' is missing when toggle the div