Redstone wire on top of hoppers and redstone blocks does not update its shape / orientation and surrounding blocks when redstone wire is placed / removed diagonally downards.
Note: In 1.12.2 the redstone wire updates client-side only, and possibly server-side not at all in any situation.
- Build the setup shown in Setup.png
- Place redstone wire on the lapis block
→ Both lamps stay on and the redstone wire on top of redstone block remains a dot
You can verify that this is not client-side only by using for example /execute if block. The lamp and redstone wire update when you cause a block update for them, for example by placing a block next (depends on wire orientation) respectively above them.