The most recent 4 or 5 versions make my 2017 5k iMac Radeon Pro 580 show up as two separate monitors duplicated, so my screen is cut in half and mirroring the other side. I can't run MW on the most recent version that actually works as it says my GPU is not supported. I noticed this probably 6 months ago when I tried to update and this happened, so I uninstalled and reverted to my last version. MW runs on Adrenaline September, but the game is duplicated on my screen twice and about half the height it should be as it is scaling as if on a portrait monitor. In AMD my monitor shows up as two separate monitors.
I know that the iMac 5k monitor has two different controllers which "fake" one screen but it was working fine before, definitely in the v18 updates. I wish I could remember which was the most recent version this was working on but I've uninstalled it to update for MW and have already done safe mode/uninstall 10 times today trying to get it to work on different versions and I'm kind of done with it for the day.
I don't know if you have resolved it but i can tell you that I had the same problem. Indeed, the last drivers wich is really well working is 18.8.2 (if you are looking for the red gaming version). Maybe the November drivers will help you, but 18.8.2 are definitively the best for now.