Standard Split button divider is not visible in high contrast mode.
Primary Split button 'split' button border is not visible in high contrast mode
Edge browser.
https://fabricweb.z5.web.core.windows.net/oufr/7.28.1/#/examples/button

Should be like this:

@micahgodbolt Thanks, I'll look into the high contrast selectors now.
While you're looking at high contrast, check out the focused state of the primary button. It looks like the internal focus ring is getting covered up by the background.
:tada:This issue was addressed in #10364, which has now been successfully released as [email protected].:tada:
Handy links:
Most helpful comment
While you're looking at high contrast, check out the focused state of the primary button. It looks like the internal focus ring is getting covered up by the background.