Player slows down during the jump when spring jumping on soul sand with soul speed boots on.
How to reproduce
1. Place blocks of soul sand in the ground where you can walk on.
2. Get any types of boots with any level of soul speed enchantment.
3. Wear the enchanted boots.
4. Walk/Sprint and Jump at the same time on the soul sand.
→ You will see that every time you jump, you revert back to having normal speed when airborne. This continues to every jump you do.
See examples in attached files...