You may also look at this topic:
https://forums.autodesk.com/t5/autocad-for-mac-forum/autocad-2018-graphics-artifacts-on-an-external-...The topic is in Russian Forum, the OP had the same problem as you with external monitor attached.
And here you can find solution for that topic, provided by Wei:
https://forums.autodesk.com/t5/autocad-for-mac-forum/autocad-2018-graphics-artifacts-on-an-external-...Or, you can try to play with LINESMOOTHING system variable I mentioned in my prev post.
Maxim
