The Dell S3221QS does NOT work with M series Macs. Period.

It doesn't matter what firmware the monitor is on.
It doesn't matter what Mac OS you are on.
It doesn't matter if it's an HDMI cable or USB-C to DP.
It doesn't matter what kind of cable or what quality you use.
It doesn't matter what settings your monitor has.
It doesn't matter what settings you Mac has.

If you buy the S3221QS to use with a Mac, you WILL experience screen flickering, and vertical lines, and other issues. 

I've wasted my money on this monitor AND on 3 different cables and adapters. Do not fall for support telling you to buy another cable! You will waste even more money on something that won't work.

Dell should frankly be ashamed of this product.