For games that use dynamic resolution scaling, such as Doom (2016), there's a trade in resolution to achieve more consistent frame-rates, but the Xbox Scorpio (the specs of which can be read here) should eliminate that, as revealed by Digital Foundry.
Doom isn't the only game to use this dynamic resolution scaling, as Halo 5: Guardians, The Witcher 3: Wild Hunt, and Gears of War 4 do the same, but the Scorpio will see this games lock to their highest resolution at all times, rather than changing for optimisation. This won't be native 4K without a patch from the developer, however, but increased GPU power will simply allow the best resolution possible.
Have you noticed severe resolution decreases in the above games, or any games for that matter?