Batch sandbox not preserved between batch iterations

How is this "working as designed", when the documentation clearly states otherwise?

// 'sandbox' (read / write): An array that can be freely used to
// store persistent data between iterations. It is recommended to
// use this instead of $_SESSION, which is unsafe if the user
// continues browsing in a separate window while the batch is processing.

same issue in here, the sanbox is not preserving the data bettween operations :(