The following will all fail with 503s:

http://localhost/helloworld?test=foo
http://localhost/helloworld?test
http://localhost/helloworld?

And I noted the following all work:

http://localhost/helloworld/?test=foo
http://localhost/helloworld/?test
http://localhost/helloworld/?

So it looks like the issue here is that mod_cluster does not recognize ? as a delimiter when checking the request context. Thus with no trailing slash it treats the query string as part of the request context and tries to check the balancer for a deployed context of /helloworld?... which doesn't exist and 503s. Adding the trailing slash, mod_cluster can properly delimit this and then sees the context just as /helloworld, which can be found on the balancer for appropriate proxying.