[bug report] cascade selector multiple choice, select any level option props.checkStrictly =True sets the parent-child node. It has no effect to deselect the association

### Element UI version
2.15.0

### OS/Browsers version
"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.146 Safari/537.36"

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/pen/

### Steps to reproduce
点击一级二级展开选中第三级的节点

### What is Expected?
多选任意一项选中哪一项，拿到哪一项的值，不包含父级节点

### What is actually happening?
多选任意一项选中一项，拿到该项整个一列的值，含父级节点