Proxy requests sometimes fails with UT001000: Connection closed

Hi all, first of all I hope that this is the correct place to report this issue (the mailing list seemed rather inactive). Please forward me to the correct place if I'm at wrong place.

We are using Undertow in a Spring Boot application. An important feature of this application is to proxy requests to a Docker container. Typically these Docker containers are R applications, running a small web-server (this are Shiny apps).
This works in most cases. However, when such an app loads many static assets (JS/CSS files) it sometimes happens that one of these requests result in a 503. Undertow then logs an exception with the `UT001000: Connection closed` message (see later for the full stack-trace).

I reduced our application to only contain the code for proxying requests and put this into a GitHub repo so that you could reproduce the problem: https://github.com/LEDfan/undertow-proxy .  The readme contains the exact steps to take to reproduce the issue (you'll need Docker (or podman) on Linux).

Note: the problem does not always happen, so please try to refresh enough times (20-30 should be enough) so that you can see the problem.

If there is a need for it, I could provide you with a pcap file containing the traffic between the browser, Spring server and Docker container for the complete page load.

I hope you can help us solve this problem. Since the problem does not occur when using the app directly (without undertow) or when using it with nginx as proxy, I think that there is some bug in undertow. Or maybe it some misconfiguration of undertow?

Please let me know if you need more information or have troubles reproducing the problem.

Thanks in advance!