If a redstone lamp is powered by an adjacent solid block powered at level 1 by a comparator that is powered at a level less than 15, placing redstone dust on top of the lamp causes it to turn off.

The "Without redstone dust" image shows that the redstone lamps are being powered by the adjacent green blocks. The length of the redstone wire causes the green blocks to be powered at level 1. Moving the comparator has no effect on the lamp because it reduces the comparator's input power level by 1 but also decreases the length of the output wire by 1.

The "With redstone dust" image shows the same circuits with a redstone dust added to the top of the lamps. In the top circuit, where the comparator is powered at level 15, the lamp continues to be lit, which is correct. But in the bottom circuit, where the comparator is powered at level 14, the lamp is dark, even though it is being powered by the green block.

Droppers and dispensers also exhibit this bug (they are activated by the green block unless they have redstone dust on top of them). Pistons and hoppers are unaffected.

This report is related to MCPE-14083, but is a little different because this one doesn't happen if the comparator is powered at level 15. It also adds droppers and dispensers to the affected redstone mechanisms.