I think all Game Pass contracts are like that - you get a one time payment and that's it. No extras if more people than usual install your game.
They aren't.
Phil Spencer says Microsoft's deals with developers are 'all over the place.'
www.pcgamer.com
"[In] certain cases, we’ll pay for the full production cost of the game. Then they get all the retail opportunity on top of Game Pass. They can go sell it on PlayStation, on Steam, and on Xbox, and on Switch. [...] Sometimes the developer’s more done with the game and it’s more just a transaction of, 'Hey, we’ll put it in Game Pass if you’ll pay us this amount of money.'"
"Others want [agreements] more based on usage and monetization in whether it’s a store monetization that gets created through transactions, or usage. We’re open [to] experimenting with many different partners, because we don’t think we have it figured out. When we started, we had a model that was all based on usage. Most of the partners said, 'Yeah, yeah, we understand that, but we don’t believe it, so just give us the money upfront.'"