The newer, larger, higher-resolution displays are certainly a wonderful development for every illustrator and designer. For plugin developers, it also means we have to create icons and images used in the UI at several sizes including 100%, 150%, 200%, 400%, etc. Unfortunately, there’s no direct support for displays running under Windows that work at 125%. The problem is that panels display at 125%, but the controls within it are still scaled at 100%.
One workaround is to simply set your display to 100% in the Windows Setting > System > Display. This makes the few problematic panels display correctly, but this probably isn’t the setting you want for everything you do on your PC.