Incase this helps anyone, this issue was fixed when I noticed NvidiaContainer was still installed and running on my system for some reason. It must have been an auto-install by windows as I used the display driver cleaner tool in safe mode, and (thought) I switched off auto-windows-driver-installs when removing my previous Nvidia GPU.
But, with the Nvidia stuff manually removed, the 2nd display doesn't dis and reconnect when it is put to sleep. Also fixed the issue that when the 2nd monitor was manually switched off but connected via display cable to the GPU, and the system was awake the whole system would freeze brielfy every few seconds (so had to be switched back on).
Anyway Nvidia bloatware is invasive!