Hello,
I am not sure this is something we support, and as per Q/A, this scenario has never been tested. Its safe to say that perhaps this will not work, since that is what you are seeing. Can you clarify if it works if you proxy both access requests and accounting frames through the same proxy server?
Otherwise, you should probably get a case started with GTAC and gather some traces and debug etc if you need it pursued further. There is nothing configuration-wise that I can think that will help here, if the source of the accounting frames is already in the switch list as you noted it is.
-Scott Keene