AppRunner request timeout / HTTP 503

I've been successfully using App Runner to deploy a containerised microservice focused around uploading large files. The Docker container hosts a Gunicorn instance with a Flask web app.

I've that for larger file uploads (which naturally take longer), App Runner will answer with an HTTP 503 response. This happens after 30 seconds so I'm assuming there is a built-in request timeout enforced by App Runner, probably by the SSL-terminating proxy or load balancer.

Unfortunately, the App Runner documentation does not mention anything regarding request timeouts or maximum body sizes, so I can only guess. Before I change the upload to use a different mechanism (probably chunked upload), or move to a service with more control (Fargate?), I'd like to first confirm my suspicion. Also perhaps there is an undocumented way to work around this problem.

And yes, I increased the timeout of the application's HTTP server (gunicorn) to 600 seconds and also tested it locally, so there is definitely something inside App Runner causing this behavior.

Any help and/or pointers are greatly appreciated.