General stuff with W10 and resolutions:
In general, here's some things to do if your games won't be in exclusive full-screen mode, especially on W10 - to try to correct that:
1. Turn the Display scaling w/in Windows back to 100%. Playing with scaling doesn't always agree with Windows, when it comes to games. Sometimes, it's as simple as that.
2. Change or make sure your NVidia Panel's scaling or AMD panel's scaling to either "No Scaling" or "100%" (i.e. normal default). Sometimes, it's the scaling giving the game some issues here.
3. Disable custom resolutions via NVidia Panel, if you're on Nvidia...or the same w/ your AMD equivalent AMD Panel program. Use the normal ones, if you're at 16:9 ratio for Widescreen here (like most gamers) - i.e. 720p, 900p, 1080p, 1440p, 2160p, etc etc.
4. Turn off some stuff running in the background, if need be - use the Taskbar Manager. Something that's running could be conflicting w/ your game. See if that does the trick.
5. Make sure your game's in-game resolution and your PC's resolution are set to the same resolution; they should match - if your Display via Windows is at 1920x1080 (1080p) in Windows, your in-game normally should match that. Otherwise, who knows what could go wrong. [shrug]
Other notes:
Older games (not built for W10) don't always agree w/ W10 anyways no matter what, so...sometimes on W10, you're stuck w/ Windowed Full-Screen or Windowed Mode (at the resolution you set it at) - see Dungeon Siege 1+2 and Space Siege, pretty much; and that doesn't always solve everything either.
Sometimes, especially for older titles - i.e. think the old Dungeon Siege 1+2 and Space Siege -you also just need an app like Fullscreenizer or Borderless Gaming to make your games into a Windowed Fullscreen, just to get them to work and render stuff properly. Run those in the background when running a game, just to knock some sense into it.
Also, check PC Gaming Wiki for your specific game and see if someone made an unofficial patch, widescreen mod, or some kind of patch that fixes a game, so it works in exclusive fullscreen.
Newer games:
If the option's there, use the in-game Dynamic Resolution or a Resolution Percentage (below or above) inside the in-game menu only, if you're trying to not match resolutions b/t your display via Windows and your actual game itself. I've had this issue w/ modern Frostbite games (i.e. ME: Andromeda and Battlefield 1) and Dishonored 2.
Say you want to run ME: Andromeda at 900p in-game (for better performance, since it runs like "meh"), but Frostbite Engine is acting up and won't full-screen b/c it wants the screen to be 1080p (it does that a lot) and the game in-game at 900p.
Don't do that, Frostbite hates that; set DSR up in-game instead. Set the main resolution to 1080p both in-game and with your display (or whatever you want - but just make them match). Go a step further and also in-game in your in-game graphics/display option, then also set the Render Resolution percentage below that to equal the render in-game for whatever framerate (if you want it do be dynamic based on performance or if you just straight-up want it rendering lower in-game no matter what). This way, you get UI always at 1080p properly looking crisp (not pixelated), even though the game might not always run at 1080p b/c you set the Percentage of rendering below that (to say whatever percentage gets you to 900p, for whatever instance of hitting a certain framerate target).
