BUG: Accessing pd.Series Multiindex with get() does not find element
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [ ] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
import pandas as pd

a = pd.DataFrame({"breed": ["stallion", "stallion", "stallion", "stallion", "pony", "pony", "pony"],
                  "stable": ["hogwarts", "hogwarts", "london", None, "hogwarts", "london", "london"],
                  "weight": [800, 900, 982, 400, 230, 300, 500]})

grouped = a.groupby(["breed", "stable"], dropna=False)["weight"].mean()
grouped = grouped.append(a.groupby("breed", dropna=False)["weight"].mean())
grouped = grouped.append(pd.Series(a["weight"].mean(), index=["all_breeds"]))

print(grouped)
print()
print(grouped["stallion", "london"])
print(grouped.get(["stallion", "london"]))
print(grouped.get(["stallion", "london"], -1))

print(f'The same? {grouped["stallion", "london"] == grouped.get(["stallion", "london"]) == grouped.get(["stallion", "london"], -1)}')
```


### Issue Description

I have a grouped pd.Series with Multiindices and can access single elements using `grouped["stallion", "london"]`, but when using `grouped.get(["stallion", "london"])` the result is `None` (or -1 when a default value is given).

### Expected Behavior

I am expecting that all three lines give me the same result:
`grouped["stallion", "london"] == grouped.get(["stallion", "london"]) == grouped.get(["stallion", "london"], -1)`

### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit           : 945c9ed766a61c7d2c0a7cbb251b6edebf9cb7d5
python           : 3.8.10.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.6.0
Version          : Darwin Kernel Version 20.6.0: Mon Aug 30 06:12:20 PDT 2021; root:xnu-7195.141.6~3/RELEASE_ARM64_T8101
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.3.4
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.1
setuptools       : 52.0.0.post20210125
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 3.0.0
IPython          : 7.24.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 2021.07.0
fastparquet      : 0.7.0
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.2
sqlalchemy       : 1.4.25
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.53.1

</details>
