The bug
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.
How to reproduce
Build the setup shown in
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.
Linked issues
Attachments
Comments 3
Could it be the case that you attached the wrong screenshot, because yours shows correct behavior.
I will attach a few screenshots that show the bugged behavior in both cases. The screenshot I already added was just used as the starting setup to produce the situation I mentioned, and I will rename it for clarity.
Similar situation in the screenshot I added. Place redstone wire on the redstone block then remove the wire on the lapis block. The unpowered lamp remains off and the wire on the redstone block remains a line rather than reverting to a dot.
To clarify the description, the redstone lamp requires a block update, not just a block placed on top of it. The wire needs a block placed above it, or for a block to be placed and removed adjacent to it's orientation (above the wire on the lapis block). Placing and removing a block on the opposite side of the unpowered lamp will not update the wire. The same applies to the situation that I mentioned. Also of note, updating the wire does not update the lamp and vice versa; they both require separate block updates. Tested in 18w05a.