Using lazychaser / laravel-nestedset, parent_id column is set to null after saving model

Question

I'm using nested set version 6.0.0 on laravel v7.30.4, I added columns to migration:

Schema::table('companies', function (Blueprint $table) {
            $table->nestedSet();
        });

And this is how I create parent and child nodes:

$parentCompany = factory(Company::class)->create();
$childrenCompanies = $parentCompany->children()->saveMany(factory(Company::class)->times(2)->make());
($childrenCompanies[0])->children()->saveMany(factory(Company::class)->times(3)->make());
($childrenCompanies[1])->children()->saveMany(factory(Company::class)->times(2)->make());

and migrated, then when I insert anything in the table, regardless of setting its parent Id or not, it is overwritten to null. _lft and _rgt are set to values but those values don't seem to make a tree.

Company model:

class Company extends Model implements \JsonSerializable
{
    use NodeTrait;

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'name'
    ];

    public function stations()
    {
        return $this->hasMany(Station::class);
    }

    public function parent()
    {
        return $this->belongsTo(Company::class, 'parent_company_id');
    }

    public function children()
    {
        return $this->hasMany(Company::class, 'parent_company_id');
    }

    public function jsonSerialize()
    {
        return [
            'id' => $this->id,
            'name' => $this->name,
            'createdAt' => $this->created_at->toAtomString(),
            'updatedAt' => $this->updated_at->toAtomString(),
            'parentCompany' => $this->parent()->get(),
        ];
    }
}

Edit: Well, somehow I fixed it :D I already had a parent_company_id in my table, and it was a foreign key to this table itself. I deleted this column and just used the nestedset parent_id column, it got fixed and now all parent Ids are setting correctly.