ASP.NET Core .NET 8 receives null form values from certain iPhone 13 Pro Max devices

I am troubleshooting a device-specific issue with multipart form data submission in my ASP.NET Core .NET 8 application.

Application details

The frontend uses vanilla JavaScript fetch with the FormData API to submit customer information and uploaded files. The backend is a C# ASP.NET Core controller action that accepts the request using FromForm model binding.

Problem

Two iPhone 13 Pro Max devices running iOS 27 fail to submit the form correctly. The browser-side diagnostic confirms that the form data contains the expected values before submission, but the backend receives null values.

The issue is inconsistent across devices. One of the two iPhone 13 Pro Max devices works against my local development server, while the other fails. some iPhone including XR and a substantial number of Android devices have submitted successfully in my testing.

I have tested both Safari and Chrome on the affected iPhone, with the same result.

Troubleshooting performed

Changed hosting from SmarterASP to AWS EC2. The issue persisted.

Changed the local development server from Kestrel to IIS Express. The issue persisted.

Tested the application locally and on the hosted environments.

Verified that the expected data exists in the browser before the request is submitted.

Technical environment

Backend: ASP.NET Core .NET 8 Frontend: Vanilla JavaScript fetch and FormData Request format: multipart/form-data Model binding: C# FromForm Affected devices: Two iPhone 13 Pro Max devices running iOS 27 Successful devices: iPhone XR and numerous Android devices

Question

What could cause multipart form data submitted from specific iPhone devices to arrive with null values in an ASP.NET Core .NET 8 controller when other iPhones and Android devices work correctly?

What diagnostics would help determine whether the problem is caused by the request generated by the browser, multipart encoding or transmission, or ASP.NET Core model binding?

I can provide the JavaScript submission code, DTO, controller action, and further diagnostic logs if needed.