Mouse buffer only works as long as the original text is still selected. Once you select some other text or remove the selection, it is gone.
It should not be the case that if you select some text AAA, press CTRL+C to copy it, then select some other text BBB, and press middle-button somewhere else it pastes AAABBB. It should paste BBB. And pressing CTRL+V should paste AAA. Can you give a usage scenario similar to this how it goes wrong for you?