BUG: SettingWithCopyError raised unsystematically (?)
- [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.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import pandas as pd

pd.options.mode.chained_assignment = "raise"


def f(df):
    res = df["a"]
    is_two = df["a"] == 2
    res.loc[is_two] = None
    return res


df1 = pd.DataFrame(dict(a=range(3)))
df2 = df1.assign(b=pd.date_range("2020-01-01", "2020-01-03"))

f(df1)  # no problem
f(df2)  # SettingWithCopyError
```

#### Problem description

The function above selects a column `a`, finds the position of values that are equal to two and sets those values to None, then returns the `a` Series. 

When a dataframe only contains the `a` column, there is no problem. When a datetime column is introduced, `SettingWithCopyError` is raised. 

#### Expected Output

Because the function `f` does not access any column besides `a`, I expect it to behave identically for both dataframes. 

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

<details>

INSTALLED VERSIONS
------------------
commit           : 9d598a5e1eee26df95b3910e3f2934890d062caa
python           : 3.8.5.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Tue Nov 10 00:10:30 PST 2020; root:xnu-6153.141.10~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.1
numpy            : 1.19.4
pytz             : 2020.4
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 47.1.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.4
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : 0.16.2
xlrd             : None
xlwt             : None
numba            : None
</details>
