[Bug Report] NavMenu 导航菜单，动态设置mode参数时，展开的二级导航错位

### Element UI version
2.15.8

### OS/Browsers version
win10

### Vue version
2.6.14

### Reproduction Link
https ://github.com/ElemeFE/element/blob/dev/FAQ.md#faq

### Steps to reproduce
1.展开二级导航
2.按钮绑定事件动态切换NavMenu mode参数
3.发现展开的二级导航错位

### What is Expected?
动态切换mode参数，展开的二级导航不应该错位

### What is actually happening?
动态切换mode参数，展开的二级导航错位