Border shapes on free from doesn't work.
As you can see the border goes missing on the upper left corner, I tried closing the app and restarting my MacBook, but it won't work.
MacBook Pro 13″, macOS 26.0
Question marked as
You're using the "feathered brush" border style. Use the "solid" border style instead.
I also recommend System Settings > Accessibility > Vision > Zoom. You can configure various ways to temporarily zoom in on the screen and see the subtle differences between those microscopic controls.