BUG: DataFrame.agg with multiple cum functions creates wrong result
- [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.

- [ ] (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
In [1]: import pandas as pd                                                     

In [2]: df1 = pd.DataFrame({ 
   ...:     'a': [3, 4, 5, 3, 5, 4, 1, 2, 3], 
   ...:     'b': [1, 3, 4, 5, 6, 5, 4, 4, 4], 
   ...:     'c': list('aabaaddce'), 
   ...:     'd': [3, 4, 5, 3, 5, 4, 1, 2, 3], 
   ...:     'e': [1, 3, 4, 5, 6, 5, 4, 4, 4], 
   ...:     'f': list('aabaaddce'), 
   ...: })                                                                      

In [3]: df1.groupby('b').agg(['cummax', 'cumsum'])                              
Out[3]: 
       a             d             e       
  cummax cumsum cummax cumsum cummax cumsum
b                                          
1      4      4      4      4      3      3
3      3      3      3      3      5      5
4      5      5      5      5      6      6
5      4      7      4      7      5     10
6      5      6      5      6      4      8
```

Cumulative functions should generate the DataFrame with the same length.

#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution]

In pandas 1.0.5, the result is

```python
In [1]: import pandas as pd                                                     

In [2]: df1 = pd.DataFrame({ 
   ...:     'a': [3, 4, 5, 3, 5, 4, 1, 2, 3], 
   ...:     'b': [1, 3, 4, 5, 6, 5, 4, 4, 4], 
   ...:     'c': list('aabaaddce'), 
   ...:     'd': [3, 4, 5, 3, 5, 4, 1, 2, 3], 
   ...:     'e': [1, 3, 4, 5, 6, 5, 4, 4, 4], 
   ...:     'f': list('aabaaddce'), 
   ...: })                                                                      

In [3]: df1.groupby('b').agg(['cummax', 'cumsum'])                              
Out[3]: 
       a             d             e       
  cummax cumsum cummax cumsum cummax cumsum
0      3      3      3      3      1      1
1      4      4      4      4      3      3
2      5      5      5      5      4      4
3      3      3      3      3      5      5
4      5      5      5      5      6      6
5      4      7      4      7      5     10
6      5      6      5      6      4      8
7      5      8      5      8      4     12
8      5     11      5     11      4     16
```

#### Expected Output

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

<details>

In [5]: pd.show_versions()                                                      
/Users/qinxuye/miniconda3/envs/test_pandas_1.1/lib/python3.7/site-packages/setuptools/distutils_patch.py:26: UserWarning: Distutils was imported before Setuptools. This usage is discouraged and may exhibit undesirable behaviors or errors. Please use Setuptools' objects directly or at least import Setuptools first.
  "Distutils was imported before Setuptools. This usage is discouraged "
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-5-3d232a07e144> in <module>
----> 1 pd.show_versions()

~/miniconda3/envs/test_pandas_1.1/lib/python3.7/site-packages/pandas/util/_print_versions.py in show_versions(as_json)
    104     """
    105     sys_info = _get_sys_info()
--> 106     deps = _get_dependency_info()
    107 
    108     if as_json:

~/miniconda3/envs/test_pandas_1.1/lib/python3.7/site-packages/pandas/util/_print_versions.py in _get_dependency_info()
     82     for modname in deps:
     83         mod = import_optional_dependency(
---> 84             modname, raise_on_missing=False, on_version="ignore"
     85         )
     86         result[modname] = _get_version(mod) if mod else None

~/miniconda3/envs/test_pandas_1.1/lib/python3.7/site-packages/pandas/compat/_optional.py in import_optional_dependency(name, extra, raise_on_missing, on_version)
     97     minimum_version = VERSIONS.get(name)
     98     if minimum_version:
---> 99         version = _get_version(module)
    100         if distutils.version.LooseVersion(version) < minimum_version:
    101             assert on_version in {"warn", "raise", "ignore"}

~/miniconda3/envs/test_pandas_1.1/lib/python3.7/site-packages/pandas/compat/_optional.py in _get_version(module)
     42 
     43     if version is None:
---> 44         raise ImportError(f"Can't determine version for {module.__name__}")
     45     return version
     46 

ImportError: Can't determine version for numba
</details>
