I'm using getters and setters for my model classes and have private properties in underscore style, I haven't had a problem with that since Phalcon 2.0.x.
You could also play with the columnMap definition.
class Language extends BaseModel
{
    public function getSource() {
        return 'language';
    }
    public function columnMap() {
        return [
            'id' => 'id',
            'code' => 'code',
            'name' => 'name',
            'native_name' => 'native_name',
            'is_enabled' => 'is_enabled',
        ];
    }
    /** @var int */
    protected $id;
    /**
     * @param int $id
     * @return $this
     */
    public function setId($id) {
        $this->id = $id;
        return $this;
    }
    /**
     * @return int
     */
    public function getId() {
        return $this->id;
    }
    /** @var string */
    protected $code;
    /**
     * @param string $code
     * @return $this
     */
    public function setCode($code) {
        $this->code = $code;
        return $this;
    }
    /**
     * @return string
     */
    public function getCode() {
        return $this->code;
    }
    /** @var string */
    protected $name;
    /**
     * @param string $name
     * @return $this
     */
    public function setName($name) {
        $this->name = $name;
        return $this;
    }
    /**
     * @return string
     */
    public function getName() {
        return $this->name;
    }
    /** @var string */
    protected $native_name;
    /**
     * @param string $native_name
     * @return $this
     */
    public function setNativeName($native_name) {
        $this->native_name = $native_name;
        return $this;
    }
    /**
     * @return string
     */
    public function getNativeName() {
        return $this->native_name;
    }
    /** @var bool */
    protected $is_enabled;
    /**
     * @param bool $is_enabled
     * @return $this
     */
    public function setIsEnabled($is_enabled) {
        $this->is_enabled = $is_enabled;
        return $this;
    }
    /**
     * @return bool
     */
    public function getIsEnabled() {
        return $this->is_enabled;
    }
    /**
     * @return bool
     */
    public function isEnabled() {
        return $this->is_enabled;
    }
}