If unplugging the display cable works though. It's most likely the TV pretending to be still on.

I have a LG TV C1 that behaves like that. While my computer monitors do not have this issue.

The TV even has a dual personality. It doesn't appear to report the same informations via EBID when powered off vs powered on.

I also have a MS Windows 10 connected to this same TV, and if I make the mistake of powering up or wake from sleep Windows before turning on the TV, then the NVIDIA GPU setup some broken resolution. And only a reboot fixes it.

So my guess is it's the TV presenting itself with different EBID when off vs powered on. And also somehow presenting itself as active on the HDMI line no matter if off or on. Changing the TV inputs also doesn't tell KDE that the display was turned off.

I haven't debugged any of it. These are just my observations.

Author here: I didn't unplug the display, I went to the settings and disabled the TV. I am not saying that I didn't do anything wrong, but I expected that disabling the TV would make the monitor the primary display and move the taskbar to it.