What do you learn by stepping through it with the febugger? Whenever something like this happens, the debugger is your best friend.
Just set a breakpoint where the Proxy sends the Notification for the first time, debug the app and trigger the send.
Then step through till it hits the Mediator to follow the path that should be taken.
Then hit the play button to set it going again and trigger the send again.
That should put you back on the Proxy, about to send. If not then whatever triggers the send has broken.
Otherwise step on through again and see where it runs off in the weeds.
Please do let us know what you find. I cannot imagine what has happened that would make the framework break.