If you want to buy Java Edition on minecraft.net, you first have to login. If you do not have a Microsoft account and click "Sign up for free", you will be brought to a registration page. If you click "Get a new email address", you can create a new outlook email without having to link a personal email to the microsoft account.

Here lies the problem: You wont receive any emails on this new outlook account until you open outlook.microsoft.com. This means, if you directly purchase Minecraft on the account without opening outlook first, outlook will never set up properly and you will never receive the confirmation email. This confirmation email is important because it contains valuable information about the purchase: Transaction ID, Order ID, etc... 

To recreate: Create a Microsoft account using a brand new outlook email address, buy a copy of Minecraft before ever opening outlook, and your confirmation email will never appear.