It's an attack vector and it won't disappear until you get rid of masternodes or find a better way for them to create anonymity.

It wouldn't necessarily need a complete redesign to shut that attack vector. All that's needed is to prevent the mixing masternode from knowing which outputs (of the mixing transaction) belong together with which inputs. The mixing peers could do a key exchange and encrypt their own outputs with random mixing peer's public key, and decrypt the others' outputs they can using their private key, and then pass clear text outputs to the mixing masternode.