Ok here's my opinion: The same thing happened to me, where it came up in "Other Purchases" with the date January 1st 2000. I talked to a few people and they too had the issue.
Theory 1: After a bit of thinking, I was positing about how we can currently "Downgrade" our Macs by downloading an older, purchased version of OS X from the App Store. As we all know with iOS devices, Apple refuses to let us downgrade to an older OS. Maybe this is their way of implementing this in macOS.
Theory 2: The Hackintosh community. Sure you can still download the Sierra installer to your Mac and then use it to install on Hackintoshes, but with Apple not allowing this to stay in purchases, it forces a Hackintosher to come up with a different way to download the file. If they want it, they will get it, but not from Apple I guess...
Again, this is what I was thinking about, nothing here is guaranteed, I just thought I would throw the idea out there!