search for activity/case with client from specific country works well only for first cliend assigned

create case.
assign to it more than one contact (unfortunately impossible on demo).
lets say that one contact has no country assigned in address. other has country X in address.
add some activity to this case.
go to advanced search.
in search set display = activity, address country = X, search for this activity type.
above activity will be found only if the first client assigned to activity (or case) has the country X. if first does not have country assigned and second has it search will not find the activity.

you can do similar search displaying case and it will be similar.