[Bug] brushMode: 'multiple' is not working

### Version

6

### Link to Minimal Reproduction

https://echarts.apache.org/examples/en/editor.html?c=scatter-simple&code=PYBwLglsB2AEC8sDeAoWsAeBBDEDOAXMgL4A0asAnjvkUmRQEYBOArngBZ0XpjDAAbRsAxEA2gHJmAUwDGYCaVgSA1tOkhFy2QOkBDZhIC65dOhbsOAWWAATaUQkBbVgMghdEig3R5pzCGlCWDEeZDDfSidhAQBlCAAvB1gAJgAGUzN0Wz0wPXEIszEARjSAOgzYAA4KgBYTQvQxGrSAdiUANjKATgBWBqyskoBmCqVWst6qgcGm7orepRqq4pnZkuKx6rLh4bXBktqtiY6O_aHi0drOspXzotKtrt37puKjyprus8z14pTJjcqilXiF5sVFrAJiCUIV7oUwJQQMkJHhZLkwP4vFkfGZULNEcjHGiMVjflk8FEYvEkkR0uTsrl8iFYbM3ptIdDQWJRmlhkojik9gyHgCUuNbsLGiEAWlxbAjrVWtzNmlIaMXiKmh9IUdvtyunyJSl6lqQv8nmUUj9pSVZUtbqbbRM0lUlL1biCzWIPXL3WUOtM4eTiBQjChiABuIA

### Steps to Reproduce

1. Click on 'box' selection on the toolbox.
2. Draw 1 box.
3. Draw another box. (The previous box is cleared)

### Current Behavior

The brush selection is 'single' even setting the brushMode: 'multiple'

### Expected Behavior

The brush selection should be 'multiple' like the behavior after the 'keep' toolbox feature.

### Environment

```markdown
- OS:Windows
- Browser: Edge
- Framework:Vue@3
```

### Any additional comments?

_No response_