BUG: DataFrame.truncate() returns incorrect data if the index is sorted in descending order

[francislan]

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

• I have confirmed this bug exists on the latest version of pandas (1.0.3)

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

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Code Sample, a copy-pastable example

df = pd.DataFrame(index=range(0, 5), data={'A': 1})
df.sort_index(ascending=False, inplace=True)
df.truncate(before=3)

Problem description

When the index is sorted in descending order, the truncate() function behaves incorrectly. In the example above, it should truncate all rows whose index is < 3. Instead it is truncating all rows whose index is > 3.
Similarly when using after.

Output

>>> df
   A
4  1
3  1
2  1
1  1
0  1
>>> df.truncate(before=3)
   A
3  1
2  1
1  1
0  1

Expected Output

>>> df.truncate(before=3)
   A
4  1
3  1

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.7.final.0
python-bits : 64
OS : Darwin
OS-release : 19.4.0
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.3
numpy : 1.18.2
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.2
setuptools : 41.2.0
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
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : 3.2.1
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None