I have not yet figured out a fix on Gnome D.E. for this issue.
However, I have been experiencing this a lot on XFCE (Zorin OS Lite) where powering on my monitors or external monitor defaults to Mirror Display.
Highly annoying.
I tried autorandr and arandr and script fixes and xorg.conf fixes... I tried a lot of suggestions around the web with either no success or only partial success.
I have finally fixed it and it was... very, very easy.
I facepalmed.
From Settings > Display >Advanced
Click
Configure new displays when connected to on.
Automatically enable profiles when new display is connected to on.
Now, I had tried this with zero success as I had already looked at the settings and it seemed confirmed in search results. But there was a detail missing in all the search results:
You need to add the profile to be enabled.
Above those buttons is the profile list. Click the add button (+)
save your current working monitor configuration by naming it and then clicking create.
That's it.
It just needed a profile for those settings to work.