Is there a problem with the $monthlyInvoices variable?

This is my job. When I run this job the online invoices query doesn't display anything, but what is intriguing is that when I swap the places of the $onlineInvoices and the $offlineInvoices variables the $onlineInvoices query works while the $offlineInvoices doesn't. It worked fine when I redefined the $monthlyInvoices again below the $offline_sales variable, But I would like to know why this occurs.

<?php

namespace App\Jobs;

use App\Models\Invoice;
use App\Models\InvoiceMonthlyStatistics;
use Carbon\Carbon;
use Carbon\CarbonPeriod;
use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldBeUnique;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Foundation\Bus\Dispatchable;
use Illuminate\Queue\InteractsWithQueue;
use Illuminate\Queue\SerializesModels;

class InvoiceMonthlyStatisticsJob implements ShouldQueue
{
    use Dispatchable, InteractsWithQueue, Queueable, SerializesModels;

    /**
     * Create a new job instance.
     *
     * @return void
     */
    public function __construct()
    {
        //
    }

    /**
     * Execute the job.
     *
     * @return void
     */
    public function handle()
    {
        $period = CarbonPeriod::create('2021-08-18', '1 month', Carbon::now()->toDateString());
        foreach ($period as $date) {
            $monthlyInvoices = Invoice::
            whereRaw('YEAR(created_at) = ?', $date->format('Y'))
                ->whereRaw('MONTH(created_at) = ?', $date->format('m'));

            $total_sales = $monthlyInvoices
                ->sum('net_price');


            $offline_sales = $monthlyInvoices
                ->where('type', '=', 'offline')
                ->sum('net_price');

        
            $online_sales = $monthlyInvoices
                ->where('type', '=', 'online')
                ->sum('net_price');


            $invoiceStats = (new InvoiceMonthlyStatistics)->create([
                'total_sales' => $total_sales,
                'online_sales' => $online_sales,
                'offline_sales' => $offline_sales,
                'month' => $date->format('m'),
                'year' => $date->format('y')
            ]);
        }
        $invoiceStats->save();
    }
}