[Bug Report] input框获取焦点问题

### VUX version
2.9.2

### OS/Browsers version
ios,google

### Vue version
2.6.10

### Code

```html

changeAccount (val) {
this.loginData.account = val;
if (val.length === 4) {
let value = `(${val.substr(0, 3)})${val.substr(-1)}`;
this.account = value;
this.$refs['Phone Number'].focus;
}
}
```

### Steps to reproduce
如果输入框有值，js为其重新赋值，再自动获取焦点时，焦点不在文字最后。

### What is Expected?
焦点在文字末尾处

### What is actually happening?
焦点在手动输入文字处