Pandas filtering on multiple columns and rows

I am trying to filter a dataframe based on a certain date and country name. I have extracted the columns I want the final dataframe to contain. I have also created a list of the country names that I want to filter on.

cols_to_keep = projects[['regionname', 'countryname', 'lendinginstr', 'totalamt', 'boardapprovaldate','location', 'GeoLocID', 'GeoLocName','Latitude', 'Longitude', 'Country', 'project_name']]
countries = ['Bosnia', 'Herzegovina', 'Croatia', 'Kosovo', 'Macedonia', 'Serbia', 'Slovenia']

Then I try to filter on the date that I want which is April 27th 1992.

projects['boardapprovaldate'] < pd.Timestamp(,4,27))

Then I try applying the date filter on the country names that I want by doing:

cols_to_keep[(cols_to_keep['boardapprovaldate'] < pd.Timestamp(,4,27))) & 

which only shows me 2 rows when I should have 12 rows

enter image description here

HOWEVER, if I try

str.contains() instead of isin()

method I can retrieve the correct results I want.

projects[(projects['boardapprovaldate'] < pd.Timestamp(,4,27))) & 
         ((projects['countryname'].str.contains('Bosnia')) | 
         (projects['countryname'].str.contains('Croatia')) | 
         (projects['countryname'].str.contains('Kosovo')) | 
         (projects['countryname'].str.contains('Macedonia')) | 
         (projects['countryname'].str.contains('Montenegro')) | 
         (projects['countryname'].str.contains('Serbia')) | 

enter image description here

Can someone please explain the difference and why this happens?

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