Redstone dust power signal won't go down slab staircase

Steps to reproduce:

1. Build a slab staircase with redstone dust on every slab

2. Add redstone dust to power the bottom-most redstone dust

3. Add blocks/slabs and redstone dust to power the top-most redstone dust

4. Add an extra slab at the top and put a lever on it so the lever connects to the dust

5. Add a lever next to the redstone dust at the bottom

Expected result:

Turning on the bottom lever will power the dust at the top of the slabs, and turning on the top lever will power dust at the bottom of the slabs

Actual result:

Turning on the bottom lever will power the dust at the top of the slabs, but turning on the top lever won't power the dust at the top of the slabs.