I found the following workaround by amending excel_api.php:

function excel_format_custom_field( $p_issue_id, $p_project_id, $p_custom_field ) {
    $t_field_id = custom_field_get_id_from_name( $p_custom_field );

    if( $t_field_id === false ) {
        return excel_prepare_string( '@' . $p_custom_field . '@' );
    }

    if( custom_field_is_linked( $t_field_id, $p_project_id ) ) {
        $t_def = custom_field_get_definition( $t_field_id );

        if ( $t_def['type'] == CUSTOM_FIELD_TYPE_NUMERIC ) {
// CZ, 2017.05.02 - commented out
//          return excel_prepare_number( string_custom_field_value( $t_def, $t_field_id, $p_issue_id ) );
// CZ, 2017.05.02 - workaround for numeric custom fields returning "number" as "<p>number</p>"
            $t_workaround_string = string_custom_field_value( $t_def, $t_field_id, $p_issue_id );
            $t_workaround_number = str_replace( array( '<', '>', 'p', '/' ), array( '', '', '', '' ), $t_workaround_string );
            return excel_prepare_number( $t_workaround_number );
        }

        return excel_prepare_string( string_custom_field_value( $t_def, $t_field_id, $p_issue_id ) );
    }

    # field is not linked to project
    return excel_prepare_string( '' );
}