When walking on half slabs placed over soul soil or soul sand while wearing soul speed enchanted boots (III), the speed enchantment does not occur. Upon removing the half slabs, the effect behaves as expected. Behavior observed in Minecraft Realms. 

Steps to reproduce

1. Place some soul soil or sand.
2. Place bottom half slabs on the soul soil or soul sand
3. Wear soul speed 3 boots
4. Walk on the slabs. The speed effect is not observed.