That is very weird. I have always used the working solution immediately after mixing it. I have never let it stand for an hour like that. So, I can't really say for sure. But I can't imagine that would have any effect. Afterall, print developer stays in trays for longer than that and development times don't have to drastically change. Rodinal can be used for developing prints (so I hear) - I wonder what happens to it after it's been in the tray for a few hours?
Did you re-do the test and mix the working solution fresh each time? Also, if you only mixed 500ml and you developed a step wedge(s), are you sure you had the correct ratio of developer to sq. in. of film? Perhaps there was too little Rodinal in the working solution so that no matter what time was used, densities would come out about the same?
I know there is a minimum amount of concentrate required for a given sq. inches of film, but I don't know it off the top of my head.
Also, the poop sheet with my Rodinal says:
"Once diluted for use, Rodinal will only keep for a short time. Therefore, it is recommended that it is only mixed immediately prior to use. The working solution is discarded after one development cycle."
Did you re-use the working solution? If so, you can't do that with Rodinal. Also, apparently sitting around for an hour would violoate the rule to use immediately.
So, I'd say use it like everyone else. Mix it right before use (like 1 minute before use or less) and then discard it. That should give you consistent results.