Despite the flickering, are you able to reach and perceive 240 Hz using HDMI 2.1 on your Alienware monitor?
I also experienced that flickering and artifact issue when I tested DP 1.4 at both 240 and 120 Hz (I also have a 4K/240 Hz monitor with QD-OLED panel, the Samsung G80SD).
With HDMI 2.1, on the other hand, I don’t have those problems (at least not at 120 Hz) and the connection is perfectly stable. However, when I select 240 Hz, the system looks and behaves as if it’s limited to 60 Hz just as I reported here, and I haven’t found an apparent solution. I thought it might be an HDMI 2.1 protocol issue, but it seems like you’re actually able to take advantage of the 240 Hz.
I’m using KDE Plasma 6.4 with Kernel 6.15.2 on NixOS.