BUG: .loc[:, 'a'] = is inplace when assigning Series, but not DataFrame

[MarcoGorelli]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

df = pd.DataFrame({'a': [1, 2], 'b': [3, 4]})
view = df[:]
df.loc[:, 'b'] = df['b']+1
print(df)
print(view)

df = pd.DataFrame({'a': [1, 2], 'b': [3, 4]})
view = df[:]
df.loc[:, 'b'] = df[['b']]+1
print(df)
print(view)

Issue Description

The above shows

   a  b
0  1  4
1  2  5
   a  b
0  1  3
1  2  4
   a  b
0  1  4
1  2  5
   a  b
0  1  4
1  2  5

Expected Behavior

   a  b
0  1  4
1  2  5
   a  b
0  1  4
1  2  5
   a  b
0  1  4
1  2  5
   a  b
0  1  4
1  2  5

Installed Versions

INSTALLED VERSIONS

commit : a38ecd5
python : 3.10.12.final.0
python-bits : 64
OS : Linux
OS-release : 5.10.102.1-microsoft-standard-WSL2
Version : #1 SMP Wed Mar 2 00:30:59 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 2.2.0.dev0+730.ga38ecd5b84
numpy : 1.25.1
pytz : 2023.3
dateutil : 2.8.2
setuptools : 67.6.1
pip : 23.1.2
Cython : None
pytest : 7.3.1
hypothesis : 6.82.4
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.16.1
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.2
bottleneck : None
dataframe-api-compat : None
fastparquet : 2023.7.0
fsspec : 2023.6.0
gcsfs : None
matplotlib : 3.7.1
numba : 0.58.1
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 12.0.1
pyreadstat : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : 1.11.0
sqlalchemy : 2.0.20
tables : None
tabulate : 0.9.0
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[phofl]

How does this behave in the multi block case?