What on earth are you on about?
Based on Epics partnership with Sony I think it would be very wise of Microsoft to stop using Unreal engine altogether. I know there are games planned for it Hell blade 2 but I dont see how the partnership with Sony doesn't impact Xbox somehow if they stay with Unreal.
Same could easily had been said about Microsoft based on Epic MegaGames partnership with Microsoft to create Gears of War. Lets not forget, Epic - a Premier technology studio was essentially a strong first party Partner of Microsoft until 2012.
Heck, Epic essentially lived in house with Microsoft covertly for YEARS to create the Gears of War franchise and I'm sure worked on various engine technologies during that partnership.
ForzaTech could essentially be derived from Unreal Engine and we wouldn't know - specifically as you can buy Unreal Engine and opt to completely own the technology averting the need to disclose
such specifics entirely. Fable Anniversary/Fable the Journey - both Unreal engine.
A number of simulator games - Unreal Engine. The latest Harry Potter Game - Unreal Engine.
Considering you can buy the engine, and are beholden to no one when renaming it/rebuilding it then calling it your own technology - you might be surprised which games are masquerading around
as some unique, cutting edge piece of technology that are actually just relabeled Unreal Engine variants.
I for one, believe Rockstar probably owns a full version of Unreal Engine - whether it uses it or not - as they can afford to just buy the engine and dissect it to learn whatever they can when they can - about
competing technologies. And then because they own the tech (because they bought it) - integrate it into their own engine. This is the type of strategy large corporate entities utilize when money is no issue.