The bug

OS: Windows 10

The structure block GUI uses | to indicate no mirroring, however the narrator cannot read this.

How to reproduce

1. Enable the narrator in the accessibility options
2. Place a structure block and right click it
3. Press Tab (↹) until the mirror button (button text |) is selected
   The narrator reads "mirror button"
4. Click on the mirror button
   As expected the narrator reads the other mirror modes, e.g. "mirror leftward arrow rightward arrow button"