A transaction which is partition-tolerant in the way you're describing requires stronger semantics than mere client acknowledgement, it requires all participant

A transaction which is partition-tolerant in the way you're describing requires stronger semantics than mere client acknowledgement, it requires all participants to engage in the consensus protocol. Unless your application joins the message broker's topology as an active member -- some systems do work this way, like Zookeeper -- it can still suffer message loss. But even if it does join, that's still not sufficient, because these systems can become unavailable during partitions, and that is definitionally incompatible with "exactly once".