I can reproduce this, but in my case the memory is freed up after a few seconds. Turning a Dynamic Map Service visbility from false to true does add about 4MB of load to the memory. But like I said, when I monitor my app, it clears up after a few seconds.
I imagine it has to do with Flash reloading the cached tiles from browser memory, but that's just a guess. Flash Garbage collection can be a bit tricky, you can never really determine when it will happen, you can't force it in a production environment. Best you can do is make sure you remove listeners and unused objects and cross your fingers. 😛