I can fully confirm the observations by eno77 for my device as well (same firmware version). I was already wondering, if others ran into same issue - so good to see, that yes, and bad to learn, that it could be a more general issue (so simple monitor exchange might not resolve the situation). In my setup, I've connected a HP microserver Gen8 via AMD Radon Pro WX3200 via DP and HP EliteBook 850 G6 via docking station via HDMI. Due to Corona, I'm working now a lot from home and the monitor should ease my life there. But currently, the randomly appearing "brightness flickering" is quite unergonomic (working with a lot of documents where bright backgrounds are the default) and also my eyes get tired over time (starting to tear).
I hope, that there are chances to correct the behavior by firmware update as in general, I'm quite happy with the monitor.

Beside, what also had been mentioned by one other user in the product page comments, speakers also sporadically make some noise when no audio is played for some time. Also annoying, but not considered as severe as above finding.

As a side note, I also bought a S3220DGF gaming monitor some time ago for my son (which had been the driver to decide for above business of similar monitor family) and also there we had quite some problems with flickering alike behavior at the beginning. Finally, we ended up to disable all advanced feature in UI (Freesync off, HDR off, ...) and after that no issue any longer. But S3221QS firmware doesn't allow so much control (e.g. Freesync can't be disabled explicitly).

As in the other thread, did you enter the Menu- Others and set DDC/CI to Off? DELL-Cares?