The main culprit here is a 8GB PowerColor Radeon RX 580 Red Dragon V2. Through the course of 6 weeks I've confirmed the problem across:
3 operating systems: W7/8.1/10, and the 10 was a completely fresh installation too
Multiple drivers: 18.12.1/2/3 as well as some slightly earlier versions I didn't keep perfect track of
2 standalone systems
And even the GPU itself has been RMA'd and replaced, however with absolutely no effect on the issue
The only completely shared factor across all tests were the screen and the HDMI cable, which are 100% not the issue as I'll explain later.
In VERY specific situations, all game-related, I'll get heavy graphical issues:
Random colored noise, either screen covering or in a continuous horizontal block from the bottom up
Blackouts and signal disconnects
Rarely the glitches will actually turn off the sound of the screen, which can and has to be turned back on via the remote
And last but not least, persistent greenouts
Most of the other glitches are temporary whereas the greenout and some blackouts are permanent until a graphics driver restart, one way or another.
As for the situations, these issues happen exclusively in games of all the workloads I've been putting on the GPU, and even in games they are far from evenly dispersed. For instance Endless Space 2, even at maximum settings and shaking the main view around there will give like one short noise flicker every 15 seconds or so.
On the other extreme of the spectrum there's Darksiders 2, where especially merely standing in Baneswood will cause all of the above glitches at such extreme frequencies there'll be more glitches than actual game footage with some driver lockup almost guaranteed to happen after a few seconds to maybe a minute or two.
Do note that stresstests and benchmarks such as AIDA/FurMark/Unigine Superposition have NOT brought forth the issue. I have gotten 2 or 3 isolated flickers out of Superposition in game mode, but that's it.
Already tried and excluded conclusions:
1. The first obvious one, bandwidth issues. There is absolutely no problem with chasing taxing loads over the cable, such as 4K videos. Some games such as X3:Albion Prelude also perform absolutely superb at maximum settings, no problems at all.
2. Problems with the screen are ruled out too, same reasons as stated above.
3. The graphics card itself seems to be fine within the bounds of what's deemed fine as per production (see same issue even after getting a replacement)
4. Problems with other hardware are also not what's going on here, as all other hardware has been exchanged, yet the exact same problems have been observed
5. It's not power issues either as I've already tried raising the power limit as per other recommendations to absolutely no effect. As a side note here, stresstests that work perfectly fine also draw more power than the games do where the problems occur.
What has "solved" the issue:
1. Setting the refresh rate to 30Hz or lower.
2. Setting the color format to something lower than 4:4:4 (the same problems are observed with either RBG or YCbCr at 4:4:4).
3. Reducing the resolution does solve the issue, however ONLY when the effective signal output resolution is reduced as well.
So for example Darksiders 2 rendered at 1920x1080 (signal resolution 1920x1080) will have no problems whatsoever. However at even 1920x1200 (signal resolution 3840x2160) the problems will be exactly as bad as rendered at 3840x2160 (signal resolution 3840x2160).
4. Underclocking the card lessens the symptoms, but does not stop them.
I have also tried a DP->HDMI adapter, however since I'm not able to select UHD-1@60Hz YCbCr 4:4:4 with this setup I can't verify whether it's an HDMI output specific problem
So at this point I've thoroughly chewed through everything I can think of, and the only conclusions I'm left with are that either this is a long standing driver problem, or it's an inherent RX 580 hardware problem that appears only under very, very specific circumstances (UHD-1@60Hz 4:4:4 in specific games like Darksiders 2). Of course I've also looked at other threads on possibly similar issues but to no avail, and I've provided more details to narrow the issue than what I've seen in other threads too.