This is with the latest 4.2.0 build on Fedora. When one process places items on the queue via the rxqueue program those items do not appear on the queue when another process queries the queue with the queued() builtin function. For instance, the following code shows zero items available on the queue when run from the command line (provided there are files in the current directory).
ls * | rxqueue
rexx -e "say queued()"