Combining two Series of dtype `Int8` converts them to `Int64`

[MarcoGorelli]

Code Sample, a copy-pastable example if possible

>>> s1 = pd.Series([91, 92, pd.NA, 94], dtype='Int8')                                                       
>>> s2 = s1.copy()                                                                                          

>>> s1 + s2  # result looks correct!                                                                                                  
0     -74
1     -72
2    <NA>
3     -68
dtype: Int8

>>> s1.combine(s2, lambda x, y: x+y)  # dtype has changed!                                                       
0     -74
1     -72
2    <NA>
3     -68
dtype: Int64

Problem description

The dtype has changed.

Expected Output

0     -74
1     -72
2    <NA>
3     -68
dtype: Int8

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 87b149f41d305fefa2a22b379b915c33b24a6d60
python : 3.7.6.final.0
python-bits : 64
OS : Linux
OS-release : 4.15.0-76-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 0.26.0.dev0+2024.g87b149f41
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.2
setuptools : 45.1.0.post20200119
Cython : 0.29.14
pytest : 5.3.5
hypothesis : 5.5.1
sphinx : 2.3.1
blosc : None
feather : None
xlsxwriter : 1.2.7
lxml.etree : 4.5.0
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.12.0
pandas_datareader: None
bs4 : 4.8.2
bottleneck : 1.3.1
fastparquet : 0.3.3
gcsfs : None
lxml.etree : 4.5.0
matplotlib : 3.1.3
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.1
pandas_gbq : None
pyarrow : 0.16.0
pytables : None
pytest : 5.3.5
pyxlsb : None
s3fs : 0.4.0
scipy : 1.4.1
sqlalchemy : 1.3.13
tables : 3.6.1
tabulate : 0.8.6
xarray : 0.15.0
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.7
numba : 0.48.0