Count Boolean values with .groupby()

[sursu]

Take the following dataframe:

df = pd.DataFrame({'ID':[1,2,1,1,2,3,4,4,4],
                   'Litmus':[False,False,True,True,False,False,True,True,True]})

I want to count the conditional number of True values, that is, I want to count the number of True values for each ID.

This works fine:
df.groupby('ID')['Litmus'].sum()

However, this does not:
df[4:7].groupby('ID')['Litmus'].sum()

To make it more interesting, this again works fine:
df[4:8].groupby('ID')['Litmus'].sum()

Bizarre behavior.

[jreback]

duplicate of #14873

pls show the actual return values, rather than 'Bizarre behavior' as well as versioning information as indicated in the template. The more complete reports are the better.