BUG: Inconsistency in week anchoring?
- [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.

---

#### Code Sample, a copy-pastable example

```python

ts = pd.Timestamp('2020-05-13 15:30:00')

freqstr = 'W-MON'
period_start1 = pd.tseries.frequencies.to_offset(freqstr).rollback(ts.normalize())

freqstr = 'W-SUN'
period_start2 = pd.Period(ts, freq=freqstr).start_time

period_start1 == period_start2

```

#### Problem description

Result is `True` where it should be `False`
I would like to raise what seem to me 2 "troubles".

#####Trouble 1
I don't understand why it is not possible to use the same `freqstr` between `to_offset` and `Period` to get the same result (which would be according my understanding `W-SUN` here)

#####Trouble 2
To get `period_start1` anchored to beginning of the week, which starts at midnight, I have to `normalize()` it, either the `Timestamp` or the `Timestamp` provided by the `rollback`.
On the other hand `rollback` documentation states: 'Rolled timestamp if not on offset'
For me the offset is at period beginning, i.e. midnight on Sunday.

#### Expected Output
I would expect the following code to give equivalent result (no `normalize` and same `freqstr`).

```python

ts = pd.Timestamp('2020-05-13 15:30:00')

freqstr = 'W-SUN'
period_start1 = pd.tseries.frequencies.to_offset(freqstr).rollback()

freqstr = 'W-SUN'
period_start2 = pd.Period(ts, freq=freqstr).start_time
```


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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-51-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : fr_FR.UTF-8
LOCALE           : fr_FR.UTF-8

pandas           : 1.0.3
numpy            : 1.16.3
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3.post20200330
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : 2.4.4
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : 0.3.3
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.0.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : 0.48.0

</details>
