I've been migrating my email archive off of Gmail gradually, because Gmail's proprietary behavior makes doing so programmatically annoyingly unreliable. I used the Purchases tab to ensure all of my receipts were migrated off/deleted. After reaching the "no purchases" point, and then returning a few months later, more purchases had shown up.

It appears that there is some sort of manual template work being done to pick up details from common order emails. So they've at times added new stores it can parse the emails from, or more versions of their emails.

And there's funny bugs too, like items where you got a pre-order price guarantee from Amazon that resulted in a small refund... will lead to the purchases tab saying the item was returned.

yeah captures a surprisingly small sample size of my purchase receipts