librenms-librenms/app/Models/PortVlan.php

<?php

namespace App\Models;

class PortVlan extends PortRelatedModel
{
    protected $table = 'ports_vlans';
    protected $primaryKey = 'port_vlan_id';
    public $timestamps = false;

    public function getUntaggedAttribute($value)
    {
        if (! $value) {
            if ($this->port && $this->vlan == $this->port->ifVlan) {
                $value = 1;
            }
        }

        return $value;
    }
}