its called a 'Rain Check' . ppl have been doing it for years when dealing with OOS items that will be continued, not OOS items that won't be replaced.
You order, pay, get item when it becomes available. If announced on the website that item was OOS and a substitute would be sent for use until stock allowances were made, that'd be cool, I'd know upfront, and would be happy that someone was trying to honor customer service, like getting a loaner car while yours is getting fixed, but the email thing is wonky, most emails go to spam nowadays, so if you don't expect one, its probably gotten deleted. Who reads their spam mail?
Can't blame Adata, they at least tried, they didn't have to. That should have been entirely a Rakuten responsibility as they are the Vendor in question, online resale or not.