I had the same problem almost year ago and there were some suggestions (click to see my topic). I wasn't able to figure out what happened. I wish you a better luck.

Maybe a demo for a game has vanished, because of owning the full version now?

Well, this is the weird thing: in the beginning, I had same thought! But the only demo of a complete title owned which I remember to have is Infinium Strike and it's still there, in hidden mode.

It is possible for the game to disappear from your Orders history and library. The order confirmation email would be the only direct evidence you have left for the purchase.

It happened to me once. But since it was a mistake/bug and I hadn't actually bought the game (well, the DLC) I'm not complaining.

Really? Stuff falling off the shelf happened... sometimes a lot (Dragonsphere, Fallout), but removed from the order history?? That's scary.

I hadn't actually placed that order, so I think it's understandable. It was the 1-dollar DLC for Thimbleweed Park. There was a promo that added it for free if you bought it during a certain period, but there was a bug/mistake and it was added to everybody who already owned the base game. I realized it because I saw a 0 € order in my order history (DLCs are not that visible in the library). After a couple of days the game was removed, as well as the order in the order history. In this case there was never an email confirmation. But as I said, it was never mine to begin with, so no complains.

when I go to my game library I see 422 but when I go to profile I see 421, not sure if it's normal but it's been there since the launch of the profile page. I still till this day wonder what game it is or not, not sure if its a demo that got removed or something?..

Never had anything removed from library. Have had games mysteriously end up "hidden" from shelf, but never removed.