Shopify Cart Quantity Bug Fix Needed

We are looking for an experienced Shopify developer to help us resolve a specific issue on an existing and fully functional e-commerce website.
The Issue

When a customer selects multiple units of the same product, the quantity is correctly displayed in the cart. However, when proceeding to checkout, the quantity changes unexpectedly to 1.

Example:

Product selected:
Rose × 4

↓

Cart:
Rose × 4 Correct

↓

Checkout:
Rose × 1 Incorrect

The expected behavior is for the checkout to maintain the same quantity selected by the customer:

Rose × 4 → Cart × 4 → Checkout × 4

We believe the issue may be related to cart.js, custom JavaScript, Shopify cart logic, theme customization, AJAX cart functionality, or the way the cart information is being transferred to checkout.
Scope of Work

We need someone who can:

Review the current Shopify cart implementation.

Identify why the quantity is being reset to 1 during checkout.

Review cart.js, AJAX/cart scripts, theme code, and any related customizations.

Check whether an app, custom script, or theme modification is interfering with the cart quantities.

Implement and test the fix.

Ensure that multiple quantities and multiple products are correctly transferred from the cart to checkout.

Avoid affecting the rest of the website's existing functionality.

The website is already operational. We are not looking for a redesign or a new Shopify store. This is a focused troubleshooting and bug-fixing task.

We are looking for someone with strong experience in Shopify, Liquid, JavaScript, AJAX Cart API, cart.js, and Shopify checkout/cart behavior.

Please include in your proposal your experience troubleshooting similar Shopify cart/checkout issues and your estimated time to diagnose and fix the problem.