BUG: Unpredictable behaviour of `Series.replace` when filling with `None` values.
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---
#### Code Sample, a copy-pastable example

```python
df = pd.DataFrame({'A': [1, 2, 3, 4, 5], 'B': [1.0, 2.0, 'a', 45.0, 0.0]})
df['B'] = df['B'].replace('a', None)
print(df)
```

#### Problem description

While replacing the values in dataframe with `None` using `.replace` method  the values to be replaced get forward filled from the previous values which is *unpredictable behaviour here*.

#### Expected Output

```
   A     B
0  1     1
1  2     2
2  3  None
3  4    45
4  5     0
```

#### Actual Output

```
   A   B
0  1   1
1  2   2
2  3   2
3  4  45
4  5   0
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-33-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.0.3
numpy            : 1.18.3
pytz             : 2019.3
dateutil         : 2.7.3
pip              : 20.0.2
setuptools       : 45.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.9.0
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : 1.2.8
numba            : None

</details>
