BUG: Series.replace(List, ...) changes the dtype of the Series to object

[EdGaere]

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

• I have confirmed this bug exists on the latest version of pandas. (v1.2.4)

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

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Code Sample, a copy-pastable example

There is a bug when calling Series.replace(List, ...), the Series gets converted to dtype 'object', which is incorrect.

Let's start with a simple example that works as expected.

df = pd.DataFrame({'a': ['abc', 'def']}, dtype="string") 

s0 = df["a"].replace('abc', 'xyz')
print(s0)

The output is as expected, and Series has maintained the type string:

0  xyz
1  def
Name: a, dtype: string

But when calling Series.replace(List, ...), Series gets converted to dtype object, which is incorrect.

s1 = df["a"].replace(['abc', 'any other string'], "xyz" )
print(s1)

Output:

0  xyz
1  def
Name: a, dtype: object

Problem description

Datatype of Series should not change when using a different argument to Series.replace().

Expected Output

0  xyz
1  def
Name: a, dtype: string

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 2cb9652
python : 3.8.3.final.0
python-bits : 64
OS : Darwin
OS-release : 19.6.0
Version : Darwin Kernel Version 19.6.0: Thu Oct 29 22:56:45 PDT 2020; root:xnu-6153.141.2.2~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 1.2.4
numpy : 1.19.4
pytz : 2020.4
dateutil : 2.8.1
pip : 21.0.1
setuptools : 56.0.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.4.1
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : 1.6.2
sqlalchemy : None
tables : None
tabulate : 0.8.7
xarray : None
xlrd : None
xlwt : None
numba : None

[mzeitlin11]

Thanks for the report @EdGaere! Persists on master, probably same underlying issue as #40732