fillna(inplace=True) does not work with columns selected by loc
not only multiple columns, but also one column.
`df.loc[df.id==123, 'num'].fillna(0, inplace=True)` doesn't work ,
but `df.loc[df.id==123, 'num'] = 123` works

why not edit the **fillna** function to adapt it  in the future.
It seems like a bug.

_Originally posted by @shuiyuejihua in https://github.com/pandas-dev/pandas/issues/14858#issuecomment-428979311_

Hi, I met with the same problem. Now that `df.loc[df.id==123, 'num'] = 123` works, which means the operation takes effect on original `df`, why fillna(inplace=True) doesn't work? This is really wired , and seems haven't been fixed. My pandas version is `0.25.3`.

Any comment or explaination are welcome, thanks!