This sounds like what my system does as well, but I made I do it intentionally. It's a KWin scrip called Autocompose. Does endeavor ship it by default?

It is caused by the KDE compositor turning off to re-direct GPU resources to the game. With the compositor off, stuff like discord or firefox on the second monitor will sometimes blank out as the compositor is no-longer working to, you know, composite the desktop.

This is not a major problem. If I want something like youtube on my second monitor, I've taken to using the play/pause button on my keyboard to toggle playback of the video, this wakes the second screen back up for me, without turning on compositing and trading in fps.

If this is the issue you are experiencing:

You can toggle compositing with ctrl+alt+f12

You could disable the automated compositor toggling, the extension that enables the feature can be found among the installed kwin scripts. This will negatively impact framerates, and especially frametimes.

One thing that could be worth checking out is whether the power supply is bad/insufficient. That could explain the GPU working when just using the desktop normally (GPU idle/low power), but when launching a game the GPU might get insufficient voltage or something causing the output to be unreliable.

Or as others have mentioned as well, maybe it's a bad display cable. To try to find the cause of the issue, perhaps try to swap the cables between the two monitors to see if the monitor having issues changes. If there is no change, try to swap the outputs used for the monitor to see if the GPU output is bad (if swapping the output causes the other monitor to have issues instead).

When the screen goes black, does the monitor complain about missing input, or is the image just black as if the GPU outputs a black image? If it is missing input, then maybe the cable/output is having issues. If the image is black then it may be the GPU having issues.

Is it only when playing this specific game, or do you have that in general, like when playing Steam games or modeling something in Blender?

For me it sounds like a hardware issue. The GPU is pretty old (age-wise, ignoring performance). Some electronics will degrade over some time and just stop working. That's my guess.

Also, two 4k-monitors sounds like torture for a card like this :D

Maybe install/ live-USB a completely other distro on a second drive and replicate this situation. Use default Ubuntu, Mint or Fedora, they have a different DE/ compositor and some other different underlying stuff.

If everything works fine, you know it's probably KDE, and if you have the same issues, it's a hardware problem and you have to repair/ replace something.

Don't forget to update me on this situation :)

Sometimes it can also be due to a wonky hdmi cable. It might be triggering re-syncs with the display.

Have you posted this topic on the EndeavourOS support forum? Unfortunately I don’t think I have a good answer for you, but they might.

Dont know much about this kind of thing, but the first place I would look at are kernel messages, using the 'dmesg' command, or the systems journal with journalctl. I haven't been an arch user in a while so maybe it's a bit different, but if you can get kernel messages you may see the video driver reporting something.

Are you sitting on a gas strut chair?

Same GPU, same problem. What fixed it for me was to use the monitor's settings to lower the DisplayPort version, but this was on a Dell monitor, so not sure if it's possible for you on the affected monitor. It might also be worth swapping cables and seeing if the Dell has the same problem.

Free sync enabled with HDMI cable? If so I'd advice turning free sink off or switching to display port