BUG: index label messed up when reset_level then unstack then sort_index
- [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.

- [x] (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
# Your code here
import pandas as pd

df1 = pd.DataFrame({'id1': [1, 2, 3, 4],
                    'id2': [3,4,1,2],
                    'id3': [1,1,1,1],
                    'x': [1,2,3,4]})
df1.set_index(['id1', 'id2', 'id3'], inplace=True)

new_levels = ['n1', 'n2', 'n3', None]
df1.index.set_levels(levels=new_levels, level='id1', inplace=True)
df1.index.set_levels(levels=new_levels, level='id2', inplace=True)

df1.unstack('id3')[('x',1)].sort_index()
```

#### Problem description

I would expect the index after the reset_level operations should be just normal index, but when doing sort_index, it seems index level ‘id1’ of row 1 is incorrectly changed to ‘n1’ from NaN and the same for level ‘id2’ of row 2. 
The result is:
id1  id2
n1   n2     4
     n3     1
n2   n1     2
n3   n1     3
Name: (x, 1), dtype: int64

#### Expected Output
id1  id2
n1   n3     1
n2   NaN    2
n3   n1     3
NaN  n2     4
Name: (x, 1), dtype: int64

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

<details>

INSTALLED VERSIONS
------------------
commit           : f2ca0a2665b2d169c97de87b8e778dbed86aea07
python           : 3.8.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252
pandas           : 1.1.1
numpy            : 1.19.0
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 47.1.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.1
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.2.2
numexpr          : None
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : 1.3.18
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
