Can't disable Auto HDR for Overwatch

It's probably your console's Auto HDR settings. I have mine disabled right now until I figure out how mine works, and I turned off the Auto HDR on my VIZIO V-Series as well.

I appreciate it. Can you please try disabling the HDR and check if there will be an improvement?

Go to Settings > General > TV & Display Options > Video modes > Un-check "AUTO HDR".

It may be cause on the X you can play Overwatch and Overwatch supports HDR.

you need hdr for 4k output

Actually, you don't.

Uhd and hdr are required for 4k output

You need UHD, in the sense that you need the TV to support 3840 x 2160 resolution, but you don't need HDR enabled to reach 4K. UHD 4K is the amount of screen resolution available measured in pixels, HDR is the ability of the TV or monitor to display more
natural colours.

Note that the Xbox One S can do HDR in games that support such a feature, but regardless of what resolution you set, all games max out at 1080P internally. If the display is 4K, it will remain in 4K, but games will only be upscaled. Only the Xbox One X
can play Enhanced titles in 4K natively.

You should be able to disable HDR, how you disable it depends entirely on your TV. Disabling HDR via the TV settings should not disable 4K screen resolution. Indeed, some Xbox One games do not support HDR, Fallout 4 on the One X can play in 4K for instance,
but it does not support (or indeed have an option for) HDR.

On my Sony Bravia Android based TV, I had to go to the main settings and enable HDR functionality. Even then, I had to set the HDR setting to 'Auto' on the HDMI port I have my console connected to. If you have HDR set to 'On' all of the time, non HDR games
will display the wrong colours, look oversaturated. With 'Auto' enabled, HDR will only operate when it detects an HDR supported game. This may not be a feature of your display however.

What make and model is your TV?

Just to be sure. Go to TV and video options and under video modes disable low latency mode, hdr10, auto HDR and Dolby Vision if you use and test games again.