When walking on the edge of soulsand the slowdown effect will not be applied. To replicate this you can place a line of soulsand one block above the ground level then walk along the edge. placing fence gates in a line next to the soulsand helps to align you so you are almost all the way off the soulsand, but still technically on it. This also effects the Soul Speed enchantment. If you enter the Soulsand straddling the edge, and never fully enter the block, soul speed will not activate, but if you continue along the edge after activating soul speed, it will not de-activate even when you switch to running along other block edges. You can replicate this by continuing the above set up with different blocks in place of soulsand. Furthermore it appears in this set up that the soul speed boots will not loose durability while you are moving along the edge (I tested this in survival). If you wish for me to provide a world showcasing this, let me know, and I will make something. Also, I have not tested this in 1.16, as I discovered it while testing the dripstone blocks.
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Don't think this is a bug, soulsand has a lower hitbox. Therefore, if you are over it and another block, you won't be on the soulsand, but floating a few pixels above. Thus you shouldn't get slowness / speed applied.

You are not over another block in this set up. The line of soulsand has air blocks next to it. That is the edge you are running on. If you look at my picture, the fence gates are floating. So you are half on soulsand half on air. Sorry if that wasn't clear.

That explanation also doesn't cover why you retain soul speed when you go from full soulsand to the edge, and continue onto non soulsand blocks, such as wool. Again if you reference my image you can see the set up go from soulsand to wool, if you start in the middle of a soulsand block, and then move to walking along the edge, you will retain your soul speed, even when you are walking in the wool area.
Hey there!
This looks to be a duplicate of MC-1133.

Ooo very likely related to that one! Good catch. It mentions nothing about soul speed being retained when entering the weird edge state while it's active, but that is probably just an additional side effect.
Thank you for your report!
We're tracking this issue in MC-1133, so this ticket is being resolved and linked as a duplicate.
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