The bug
Entities are now slowed down when standing on soul sand, but not being sunken into it. This was not the case in 1.14.4.
This issue also causes items to stop just in front of water bubble elevators sometimes, since they're slowed down too early.
Videos
Video of the bug happening in 19w46b:
Video of the bug not happening in 1.14.4:
Attachments
Comments

Can confirm. The items probably sink too far down into the soul sand and thus aren't affected by the bubble column anymore.

This is not because they sink down but because they are partly outside of the water. The item gets significantly slowed down from the soul sand slow movement effect och stops before it is fully inside of the water block. So since it is partly in water and partly not it get an upforce from the bubble column but a down force from being in the air so it just stays in place. I'm unsure of a good solution right now.
If I understand you correctly, wouldn't the easiest fix for the issue be to change soul sand so that it can only slow entities down if their hitbox is inside of the block, not if the hitbox is only touching the block?
This issue affects all kinds of entities. I have updated the ticket accordingly.

Confirmed for 1.15-pre2.

It looks like this is fixed in 1.15-pre3, can someone confirm?

Okay, I was mistaken, this is still in pre4. Really should be fixed before the full release as it breaks a lot of contraptions.