so i had set up a very gentle ramp, going from 1 layer of snow (to carpet) to a the full 8 layers, with soul sand being below the snow. I then ran up the ramp while wearing boots with varying levels of Soul Speed (the bug was the same for every level of the enchantment) 

The Soul Speed enchantment functioned, increasing my movement speed over the soul sand for 1 and 2 layers of snow (and on the carpet), and did not work for 3 layers through 8 layers.

However, on layers 3 through 5, I was slowed, to the same speed i would be if i were simply walking on soul sand, without wearing boots enchanted with Soul Speed

What I expected to happen was...:
Either not be faster, but not be slower either on layers 3 through 5, or to be faster, as caused by the Soul Speed enchant

What actually happened was...:
Was slowed by soul sand when 3-5 layers of snow above it, even while wearing boots enchanted with Soul Speed

Steps to Reproduce:
Be between 3 and 5 layers of snow above a piece of soul sand, wear Soul Speed boots I guess, and walk around. I do not know if this happens on other blocks with similar heights.