[bug report] cascade selector, lazload status echo, reassignment echo failed after more than 1 second, no problem within one second

### Element UI version
2.12.0

### OS/Browsers version
Mac/Chrome版本 77.0.3865.120（正式版本）

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/hua2/pen/jOOBVZw?editors=1111

### Steps to reproduce
created 后模拟 2秒后进行重新赋值

### What is Expected?
无论何时重新赋值都进行lazyload 回显

### What is actually happening?
一秒内没问题，超过一秒不进行回显