Can you replicate the issue on a PC running the supported Windows OS?

Dell monitor, Apple FAQ
None of the Dell D, E, G, P, S, SE, Alienware monitors have any macOS support from Dell. As a consideration to macOS users, on some of these monitors we did provide a firmware update that can be run from the macOS.

Today I bought a USB C x DVI cable and so far the problem has NOT happened. I'll test a few more days and come back to tell you what happened.

Great! For other macOS users, can you provide a web link to the specific USB-C to DVI cable that you purchased?

Hi! I'm using the screen with two dell notebooks XPS13 and XPS15 on ubuntu linux and tried all three USB-C to HDMI adapters I got lying around + different cables and experience the same. So it's not because of the Mac. Actually you can search on youtube for S3221QS flicker and will find a few videos with quite a few more commenters who have the same problems. For me, it's caused by grey-to-dark-grey transitions (fade-in and -out effects or on scrolling).

Setting the "FPS" game preset mode worked to mitigate the problem reliably. I would have tried to update the firmware but it looks like I need to setup windows to do this. Hope this helps.

P.S.: Also, when I have the OSD open, and move the offending window (Slack in my case) under the OSD, then the problem doesn't appear as long as the OSD is open. Once the OSD closes itself, the whole thing starts again. This can't be caused by the adapter cable and looks absolutely like image-conditioning by the panel gone wrong..

Update:
I tried a fourth adapter (this time from Dell - the one that was included with the XPS15) and the flickering disappeared