When you walk onto soul sand/soul soil from a different block, the speed boost you get is heavily delayed. Your walking speed is kept normal, Soul Sand dosen't even slow you, it just dosen't affect your speed at all. You also still see the neat little soul particles when you walk around. However, you won't get speed boosted for a while. This could very likely tie into
How to replicate:
-Set up a platform of soul sand and/or soul soil
-Equip a set of boots enchanted with soul speed
-Walk onto the soul sand/soil
-You gain a speed boost as soon as your feet touch the soul sand/soil, indicated by the FOV change.
-You gain no speed boost until around 20 to 40 seconds after you've walked on the soul sand/soil.