Looking at the CommandAwareRpcDispatcher.handle(Message) (line 139):
cmd = (ReplicableCommand) req_marshaller.objectFromBuffer(req.getBuffer(), req.getOffset(), req.getLength());

Message.getBuffer() already returns a copy of the message buffer that takes into account the offset and length. So, it would seem the above code will return a subset of the desired buffer if the offset > 0.

Either this code should operate on the raw buffer (i.e. req.getRawBuffer()), or pass 0 as the offset to objectFromBuffer(...).