With respect to the behaviour exhibited with iCUE (and I have a suspicion what's happening here is related to the other flickering I am observing, which I will mention below), I wish I had a way of showing it, as it's startlingly obvious to experience, but hard to capture in words or with video. I've maxed the brightness in the following clip. When I click on Steam, flickering vanishes, and when I click on iCUE it returns. Furthermore, as previously mentioned, dragging iCUE around gives a 60Hz 'feel' and immediately dragging Steam gives that buttery-smooth high-refresh rate feeling. At around the 20 second point in 1080p, you should be able to see the flickering, it's very obvious sitting in front of it.
I've now started noticing flickering in menus of games, non-rendered cut-scenes, etc (and in rare cases, in-game, although iirc this has only happened in borderless windowed mode, which is an edge case). It's intermittent though, I cannot detect any specific reason why it does it. I am, however, presuming that whatever is happening with iCUE is happening wherever else the flickering appears.
I know I'm repeating myself, but just to be clear, this only happens with adaptive sync on.