Ok. So I narrowed it down to the kscreen daemon messing up. It seems to be plagued since ages, when it comes to dual monitor setups and just recently, another problem emerged:
https://bugs.kde.org/show_bug.cgi?id=376341
I fixed it manually: In .local/share/kscreen there are three config files. In one of them I found (notice the changed values)
[
{
"enabled": true,
"id": "7938d2767368e7d0741d5b5df4548801",
"metadata": {
"fullname": "xrandr-Samsung Electric Company-SAMSUNG-16778753",
"name": "HDMI-0"
},
"mode": {
"refresh": 60,
"size": {
"height": 1080,
"width": 1920
}
},
"overscan": 0,
"pos": {
"x": 3840,
"y": 0
},
"primary": false,
"rgbrange": 0,
"rotation": 1,
"scale": 1,
"vrrpolicy": 2
},
{
"enabled": ~~false~~ **true**,
"id": "66f3667408b1cffa2e742224b70eda61",
"metadata": {
"fullname": "xrandr-LG Electronics-38GN950-007NTNHGV535",
"name": "DP-4"
},
"mode": {
"refresh": 119.98229217529297,
"size": {
"height": 1600,
"width": 3840
}
},
"overscan": 0,
"pos": {
"x": 0,
"y": 0
},
"primary": ~~false~~ **true**,
"rgbrange": 0,
"rotation": 1,
"scale": 1,
"vrrpolicy": 2
}
]
Now, everything works as expected, the energy saving bug is gone.