For a nether hub I am building (on a server), there are slabs on top of soul sand so that players with Soul Speed boots are able to run on it. An image of it is attached.

Since soul sand underneath slabs slows down players as intended, I expected Soul Speed to work even on top of slabs. However, it instead slows you down as if you did not have the Soul Speed enchantment. When I remove the slabs and run directly on the soul sand, Soul Speed works as intended.

For some reason, Soul Speed works if there is a trapdoor on top of soul sand, but does not work if there is a daylight detector or slab.

To reproduce:
1. Place soul sand on the ground.
2. Put bottom slabs on top of the soul sand.
3. Equip boots enchanted with Soul Speed.
4. Walk on top of the slabs.