When searching for in ezcountry fieldtype, if the multiselect option is checked and many (10+?) countries are selected, only the first ones will be searchable/found.

    $query = new SearchQuery();
    $query->criterion = new Criterion\Field( "countries" , Criterion\Operator::CONTAINS, $countryName );

sort_key_string is defined as varchar(255) and, in case of eZ County datatype will receive a "comma separated lowercase country names"

// Sort key should contain comma separated lowercased country names.
$fieldDef->defaultValue->sortKey = $storageDef->dataText5;

The problem is, if you have multiple select enabled in eZ Country, there may be more than 255 characters and the sort_key_string will be truncated, leaving out part of the countries