[bug report] when the table is frozen, the hover style is wrong, and the hover row is added incorrectly

### Element UI version
2.15.3

### OS/Browsers version
mac / chrome 版本 91.0.4472.114 (正式版本) (x86_64)

### Vue version
2.6.14

### Reproduction Link
https://jsfiddle.net/u7qpL50t/

### Steps to reproduce
table組件內放一個table組件
並且啟用fixed 凍結外層表格
打開內層後
hover-row的添加就會有問題

### What is Expected?
應該與未添加fixed時樣式一樣 hover row正確顯示highlight

### What is actually happening?
打開內層表格時 hover樣式亂跳
如在第五列 第一或其他列出現highlight