I see this kind of thread hanging around + my "app" doesn't exit:

• https://gist.github.com/alesj/99157a4f0f6c7cd94447a206b16cd702#file-gistfile1-txt-L20

It's due to "this.cleanupExecutor = cleanupExecutor;" being set to false ...
See see org.jboss.resteasy.microprofile.client.RestClientBuilderImpl#238

public ResteasyClientBuilder executorService(ExecutorService executorService)
{
return asyncExecutor(executorService, false);

//...

// this is rest easy default
    ExecutorService executorService = this.executorService != null ? this.executorService : Executors.newFixedThreadPool(10);

    ExecutorService executor = AsyncInvocationInterceptorHandler.wrapExecutorService(executorService);
    resteasyClientBuilder.executorService(executor);

As we can see new executorService is created, added as asyncExecutor, but with cleanupExecutor==false.
Hence the executor is never shutdown in client's close().

If I use and control own executor, it all works (exists) as expected.

I use some CompletionStage as return type in my app's REST API, which uses this executor – hence "leaking" the thread.
If no async REST API is actually used, it's all fine – probably why this didn't show-up before.