Sometimes failed to create scatter operator after splitting

## Bug Report

cc https://github.com/pingcap/tidb/issues/31034

### What did you do?

Execute `ScatterRegions` over some fresh regions created by `BatchSplitRegion`.

### What did you expect to see?

The regions are scattered and balanced.

### What did you see instead?

Some of the operators failed to be created because of `unhealthy`. And the final region isn't balanced.

### What version of PD are you using (`pd-server -V`)?

```
/var/lib/pd/log # /pd-server -V
Release Version: v5.4.0-nightly
Edition: Community
Git Commit Hash: ae23d409c528a836ae6d98cd174689c38ad19f2d
Git Branch: heads/refs/tags/v5.4.0-nightly
UTC Build Time: 2021-12-30 12:11:23
```

### Note

(The details of metric and cluster info TBD)