mutateFormDataBeforeSave doesn't initially save to.

  protected function mutateFormDataBeforeSave(array $data): array
    {
         $fullname_array=array($data['prefix'],$data['fname'],$data['lname'], $data['suffix'], $data['alias']);
         $data['fullname'] = implode(" ",$fullname_array);
         return $data;
    }

class Persoon extends Model
{
    use HasFactory;
    protected $fillable = ['prefix', 'fname', 'lname', 'suffix', 'alias', 'fullname'];
}