When you book, it will always take the points most restricted first, so banked first.
If you book the Aug 2021 first, then once it takes the banked points, they will be there unless you cancel,
If does not reallocate on its own so if you want banked to be used in May, you have to book that one first
If you did book May after August trip, and the Aug trip gets canceled before May, then you can get them switched,
But, if you went in May and then in June had to cancel August, they can’t apply the banked points to a trip already completed.