The answer can be found on GitHub here. The problem was following (answer from GitHub by Ladicek user):

... This is because your test sends the message and immediately stops; it doesn't wait for the processing to finish. When Quarkus stops, an exception like this can happen very easily (as well as a bunch of other exceptions). If you for example add Thread.sleep(5000) to the end of the @Test method, the exception isn't thrown. ...