The fix is changing core/commands/IssueAddCommand.php

From:

if( $t_version_id != 0 ) {
        if( !access_has_project_level( config_get( 'report_issues_for_unreleased_versions_threshold' ) ) ) {
                throw new ClientException(
                        'User not allowed to assign unreleased versions',
                        ERROR_INVALID_FIELD_VALUE,
                        ['version']
                );
        }
        $this->issue->version = version_get_field( $t_version_id, 'version' );
}

To something like:

if( $t_version_id != 0 ) {
        if( !version_is_released( $t_version_id ) && !access_has_project_level( config_get( 'report_issues_for_unreleased_versions_threshold' ) ) ) {
                throw new ClientException(
                        'User not allowed to assign unreleased versions',
                        ERROR_INVALID_FIELD_VALUE,
                        ['version']
                );
        }
        $this->issue->version = version_get_field( $t_version_id, 'version' );
}

EDIT (dregad): add markdown for proper code indentation