I found a solution on these forums and it worked for me!!

@IXTOXICITYIX
I just found a fix that worked for me see if it will work for you but you have to turn off the 4k-144Hz option the monitor it self. Once you have done that it will correct it self to be full-screen.

The 4k-144Hz is under the system setting on the monitor.

So you need to go into the menu on the monitor itself

• Click bottom joystick button on monitor
• Click it again to go to "Menu"
• Scroll down to "System", Click joystick to enter section
• Scroll down to "4k-144Hz Mode", Click joystick to edit
• Change to "Off"

I had to do this while in the game. If you do this outside the game it will switch back to "On"

But now I'm at set in-game to 3820x2160 (native) and 120Hx (native). With the "4k 144Hz Mode" set to on I didn't have the "(native)" label on the resolution and refresh rate in-game. With it "Off" I now have that label.

I found a solution on these forums and it worked for me!!

@IXTOXICITYIX
I just found a fix that worked for me see if it will work for you but you have to turn off the 4k-144Hz option the monitor it self. Once you have done that it will correct it self to be full-screen.

The 4k-144Hz is under the system setting on the monitor.

So you need to go into the menu on the monitor itself

• Click bottom joystick button on monitor
• Click it again to go to "Menu"
• Scroll down to "System", Click joystick to enter section
• Scroll down to "4k-144Hz Mode", Click joystick to edit
• Change to "Off"

I had to do this while in the game. If you do this outside the game it will switch back to "On"

But now I'm at set in-game to 3820x2160 (native) and 120Hx (native). With the "4k 144Hz Mode" set to on I didn't have the "(native)" label on the resolution and refresh rate in-game. With it "Off" I now have that label.

@noswall2112 thanks! this worked for me as well - I was looking all over for a fix for this!

Same issue here with the letterbox format at Fullscreen or Borderless on a 4k HDR monitor.

Monitor: Acer Predator XB273K GP (3840x2160, 120Hz, HDR on, G-Sync on)
Video Card: Nvidia GeForce GTX 2080 Ti (with current GeForce driver: 457.30)
CPU: AMD Ryzen 7 3800X
RAM: 16 GB
Motherboard: Asus TUF Gaming X570-Plus (Wi-Fi)
Hard drive: SSD
OS: Windows 10 Pro 64-bit, DX 12

If I have the in-game setting set to "Borderless" or "Fullscreen" I get the letterbox format. If I set it to "Windowed" the letterbox is gone. I can adjust the window to almost fullscreen but I still have the window panes top bar. If I maximize the window the letterbox comes back.

I've tried different resolutions, refresh rates, HDR on/off in the game and system/OS level and nothing has removed the letterbox. I tried forcing the scaling mode in the Nvidia Control Panel to override the games/programs and that didn't help either.