As best i can tell, what you suggest happens on one to one messages. But there seems to be something in the way iMessage stores group messages, and use whats stored when responding, thats the problem.

If i am to guess, the client-server protocol only sends a single message id, and thus when one part of a group fails, the server has to invalidate the whole group.

And Apple would be loath to do that as it would perhaps be seen as an inconvenience to the user.