Looks like a Windows or display driver bug, especially if it's affecting more than one program and only in the notification icons.
Notification icons are drawn by Windows. Programs just give Windows the icon bitmap, and it then holds on to it and draws it when it needs to, with no further involvement from the program it came from, until they tell it to change the icon (which Opus never does).
The Windows shell still has a lot of icon caching and DPI changing bugs. Clearing the icon cache may fix things. Rebooting after changing screens may also fix things if this is one of several bugs that happen when parts of Windows are not running at the system DPI (the primary monitor's DPI at boot time), and if changing screens is also triggering a DPI change.
Most of the other icons in your screenshot have very dark outlines, like the alpha blending is not being done correctly. I suspect this is affecting more than just the two programs, it's just more visible with some icons than others.