If I connect my Ubuntu laptop with HDMI to a secondary display configured to mirror the screen, then once I unplug the HDMI cable, my desktop wallpaper becomes black. I can still see the icons and I can fix the wallpaper by setting it again, but every time I unplug my HDMI cable, the wallpaper becomes black again. All other display modes don't have this issue: Extend and Second Display Only don't change the wallpaper, and I don't get the issue either when switching from Mirror to the other modes with Super+P.
What might be the cause, or how can I begin to troubleshoot this?
I wondered if the wallpaper got moved to off screen or something, but even if I select to tile or stretch the wallpaper, I still get the black screen. My laptop has an Nvidia GPU, I wonder if that may be relevant. I haven't experienced other issues with graphics / games / CUDA computation though.
System is running Ubuntu 20.04 LTS.Kernel version is 5.15.0-105-generic.Nvidia driver number is 535.171.04Desktop environment is Unity 7.5.0.Hardware is Dell Inspiron 7577, which has an Intel(R) Core(TM) i5-7300HQ CPU and a Nvidia GTX 1060 Max-Q GPU.