replace method does't work with string type Series
#### Code Sample, a copy-pastable example if possible

```python
>>>pd.Series(['A','B']).replace(r'.','C',regex=True)
0    C
1    C
dtype: object
pd.Series(['A','B']).astype('string').replace(r'.','C',regex=True)
0    A
1    B
dtype: string
```
#### Problem description

It seems that replace doesn't work with the string type Series.
Why these two codes return different results?