I have no idea how long this bug has been in the game for, so please give me an update on it. For now I know it affects 1.12 and is probably a long-standing bug but that is it.

The redstone lamp with the slab above it is the one to focus on. As you can see, the redstone is placed in such a way that the redstone lamp with the slab above it is not powered (Fig. 1) (intentional).

However, placing the sandstone block to change the direction of the wire to point directly towards the lamp will not power the redstone lamp (Fig. 2) and only updating the lamp will fix it (Fig. 3 - I simply replaced the redstone).

The steps of reproduction are simple:

1. As shown in Fig. 1, build a 2x2 square of redstone lamps and some redstone, built exactly as shown.
2. Then change the connectivity by placing a block (as shown in Fig. 2). You should find the redstone lamp(s) do not update.
3. Give the redstone lamps a block update and they will power.

Edit: it should probably be worth noting that changing the connectivity again (as shown in Fig. 4) will also not update the redstone lamps.