Upgrading to Play 2.9 leads to 503 - upstream connect error or disconnect/reset before headers. reset reason: connection termination for Test Clients

[sedhha]

After upgrading to Play 2.9, I had some of my integration test suites written in java that involves binary uploads. I am observing that the upgrade always leads to 503 - upstream connect error however downgrading it back to 2.8 just works as expected.

java.lang.AssertionError: expectation "expectNextCount(1)" failed (expected: count = 1; actual: counted = 0; signal: onError(org.springframework.web.reactive.function.client.WebClientResponseException$ServiceUnavailable: 503 Service Unavailable from POST 

The point to note is that this error only reproduces when running Java test suites and not otherwise when using swagger, postman or normal curl as well. I suspect it has to do something HTTP version but when changing HTTP version, unfortunately I am not able to reproduce the error. Is there any http duration or rate liit that's being set? I am using play 3.0.3.

Weirdly enough this happens only with the Java automation test suites and nothing else. Like if I invoke the same API from swagger, postman or any other service it just works. The Java Test Suite is using Java 17 and is also using self signed https certificates. Can anyone help me figure out what maybe going wrong here and what all should I try to fix this?