When you setblock a lit redstone block without an power source powering it, it will turn off, unlike all other blocks. Lit furnaces, repeaters, comparators, wire, etc. all remain lit, but lamps turn off.
- Make sure you stand near nothing that could power the block in your feet.
The reason this happens is because of a check when a lit redstone lamp is setblock-ed:
However, this check does not exist for:
- pistons (extended)
- dispensers, droppers (triggered)
- comparators, doors, fence gates (powered)
- furnace, repeater (lit block variant)
- redstone torches (unlit block variant)