The bug
If you jump while running with Soul Speed boots on soul sand, the player will slow down significantly when landing on the soul sand.
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Yeah, I've tested more and it seems to affect soul sand only, thx =).
Thank you for your report!
We're actually already tracking this issue in MC-174469, so I've resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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Can confirm, but only for soul sand. I'll remove soul soil from this ticket, jumping on it doesn't slow the player down as much.