I tried doing many sales today for All Inventory, trying to get all of my items, I have about 8k of my 20k items that would not go on sale.
I tried to manually add the listing IDs for the ones that didn't go on sale. After the sale processing completed, I looked at the items I chose and they were marked "ALREADY DISCOUNTED" . It then added 0 new items to the sale, despite some of the items I chose not being discounted. Here is a screenshot showing that:

There is no way for me to add 8k of my items to the sale because it thinks they are already discounted.
This is a major issue.