Why am I receiving 413 errors from AWS App Runner when no rules are configured in the associated Web ACL?

Hello,

I am currently dealing with a peculiar situation involving AWS App Runner and AWS Web ACL (WAF). Specifically, I am receiving 413 errors (Payload Too Large) from my App Runner instance when it's associated with a Web ACL, even if no rules are established within the WAF. This occurs with any files that are larger than 1MB.

The WAF logs indicate that the incoming requests are permitted. However, these requests never make it to my App Runner application (confirmed via application request logs and app runner metrics).

These are the response headers with the WAF associated:

HTTP/1.1 413 Payload Too Large
content-length: 17
content-type: text/plain
date: Sat, 01 Jul 2023 23:08:19 GMT
server: envoy
connection: close

Whereas as soon as I disassociate the WAF from the App Runner instance, requests reach my application again and receive the expected 200 OK response, with the following headers:

HTTP/1.1 200 OK
access-control-allow-origin: [redacted]
content-length: 15298
content-type: application/json; charset=utf-8
date: Sat, 01 Jul 2023 23:09:19 GMT
x-powered-by: Express
x-envoy-upstream-service-time: 25723
server: envoy
connection: close

From some testing, it also seems weirdly like when files larger than 250KB but smaller than 1MB are uploaded something similar but different is happening - they're allowed by the WAF but don't reach my application. But a 500 is returned rather than a 413.

I assume this is a bug, but otherwise would love any insight into why this is happening, or how I could address this issue? I would greatly appreciate your help.