TiKV region balancing does not work

## Bug Report

### What did you do?

1. Create a TiKV cluster on k8s. tikv storage size is 2Ti with 30 replicas.
2. Insert 10TB of data.
3. Wait until leader/region balancing finishes.
4. Insert another 10TB of data

### What did you expect to see?

Leader/region balancing works.

### What did you see instead?

Leader balancing worked, but region balancing did not work.

PD panel shows that there are 10 LowSpace stores.

Many stores have enough available storage.

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

v8.5.2