The Netty org.jboss.resteasy.test.AsyncIOTest is very flaky. There is an issue with determining if something is running on the IO thread or not. This is determined with a utility method that does:

public static boolean isIoThread() {
    return Thread.currentThread() instanceof FastThreadLocalThread;
}

It looks like the expectation for the test is that it it's not supposed to be on the IO thread, a io.netty.util.concurrent.FastThreadLocalThread thread, then it's expected to be on the thread used in a CompletableFuture.supplyAsync(). This does not always seem to be true for some reason. Whether there is a real race condition on when the current thread would be changed or there is an issue with the test alone is undetermined.

I did however, remove all other invocations of endpoint checks from the test and left one which fails every time. This could be an indicator on the test just being bad. This needs to be looked at closer though.