Is there any difference in the graphics card configurations or the display settings between your two partitions?
No, both run at 3840x2160

Do you have any graphics settings in the game that are different? Are you using Fullscreen or Windowed mode?
No, both use the same settings. I tried windowed mode and fullscreen.

Can you provide the specs for your iMac?
Model Name: iMac
Model Identifier: iMac18,1
Processor Name: Intel Core i7
Processor Speed: 4,01 GHz
Radeon RX 580: Radeon RX 580
Displays Resolution: 3840x2160 (2160p 4K UHD - Ultra High Definition)
UI Looks like: 1920 x 1080 @ 60 Hz