The data will be committed, but you may get a subsequent read that has the previous value, before it is overwritten by the final value. This would only occur if

The data will be committed, but you may get a subsequent read that has the previous value, before it is overwritten by the final value. This would only occur if the shard where the commit is occurring fails, while the promoted shard replays the transaction before the final commit is propagated to the user. Since the shard as failed is impossible for a single transaction to experience this behavior, but an outside observer would be able to see this behavior. Transactionally everything would still be isolated, but outside the transaction you would see behavior you won't expect.