Why does my Django app work locally but return 400 Bad Request when deployed?

Django 5 project deployed to a cloud host (Render). Locally I run python manage.py runserver with DEBUG=True and everything works. In production I set DEBUG=False and pointed my domain at the service.

Visiting my production domain should serve the site exactly like it does locally.

Every request to the production URL returns 400 Bad Request, with nothing useful in the browser. Logs show the request arriving but Django rejecting it immediately.