I have two monitors identified as LG Ultragear 27GL850. A bit different model in fact, but basically the same. Both connected via DP. With DP connection it is possible that when you're turning a monitor on/off, Windows (10) detects it and changes screen configuration accordingly. And that was exactly so with my older monitors.
However, now with the new model I've mentioned, it works in a strange way. When I turn one of the monitors off, it changes nothing. Only when I turn the monitor on again, Windows detects it being disconnected and immediately reconnected, changing the config to one screen and then instantly to two screens again. This is kind of annoying because I often want to turn off the secondary screen, when I don't need it, and not be infuriated with programs opening or maximizing on the wrong screen, which Windows thinks is still there, or with the cursor getting lost. webpage
The graphics is 6900XT, both it and the monitors support DP 1.4. So is there a way to make the system actually detect monitor disconnection?
I don't have any experience running two monitors off of a single video card, but here is what I would try:
(1) Swap the DP connections at the video card. See if the behavior changes to the other monitor.
(2) Update your graphics driver to the latest version.
(3) Look into Windows settings (using Windows 10 or 11?) for primary / secondary monitor behavior. (Windows / System / Display / Multiple Displays)
(4) Check your motherboard BIOS settings to see if you are running PCIe 3.0 vs. 4.0