You should be testing the U3818DW after resetting to Factory settings, (OSD Menu -Other -Factory Reset), 3840x1600 60Hz, page 45 in the User's Guide. Post a video of the U3818DW within windows playing a game. Is your video card driver sending 10 bit or 8 bit? If 10, can you force it to 8 and retest?

We also need more data =
* Was the U3818DW purchased directly from Dell or some reseller? And when?
* What is the 20 digit alpha-numeric U3818DW serial number and A0x revision?
* To what computer model is the U3818DW attached?
* Have you tested on a second computer? If so, what are its specifications?
* What operating system is installed on the computer?
* What video card/GPU is installed in the computer?
* What video OUT ports are on the video card/GPU?
* List all U3818DW cables and adapters attached to the computer and the ports they are using and how they are connecting to the U3818DW

I have the exact same problem.

Tested with Win10 and MacOS High Sierra - both 64-bit and tried with both Nvidia, Intel.

Also tried with forced 10-bit and 8-bit, did a factory reset and tried with HDMI and DP.

It's not the first person to have this issue either, but Dell just keeps claiming it's "supposed" to be like this.

The issue happens if contrast is over 51% and it becomes much worse if setting response time to "fast" (5ms). The video below are on "normal".

Video: https://youtu.be/MHX9rYdhqRE