[Bug Report] Remote Search Select focuses wrong element when using arrow - Buttons.

### Element UI version
2.15.1

### OS/Browsers version
Widows / Google Chrome Version 88.0.4324.190

### Vue version
2.6.12

### Reproduction Link
https://element.eleme.io/#/en-US/component/select?jsfiddle#remote-search

### Steps to reproduce
- Type "a" in the remote Search Input
- wait for the Elements to show up
- now type "r" behind the "a"
- wait (...)
- now delete the "r" so only the "a" stays in the input - field
- now use the Arrow Buttons (up and down) to select some elements.

### What is Expected?
The Elements are selected in the correct order when using the arrow - buttons.

### What is actually happening?
Some elements will be skipped.