This had been reported by me already in #9823 but was never solved. As the Minecraft Wiki says (http://minecraft.gamepedia.com/Soul_Sand#Usage), when the Soul Sand is placed over Packed Ice, Ice, Slime Blocks, ect. it is supposed to slow the player even more than sole Sould Sand.
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This no longer occurs in 1.16, but that may only be because soul sand was given a full collision box. The full soul sand collision box is tracked as a bug at MCPE-87458.
This no longer occurs in 1.16, but that may only be because soul sand was given a full collision box. The full soul sand collision box is tracked as a bug at MCPE-87458.