Items floating up through unsupported waterlogged scaffolding seem to think that they are stuck inside a block when they reach the bottom of the scaffolding and get flung out. It doesn't seem to be a problem for the top part of the unsupported scaffolding because they just float on through if you throw the item into the middle of the scaffolding.

my guess: It might have something to do with the rules of the scaffolding block that lets a player crouch and descend through the top layer of unsupported scaffolding but not the bottom layer. 

reproduce steps:

1. Create a 3x3x3 of water
2. Place scaffolding across the top layer of the 3x3x3 so they are waterlogged but unsupported directly(need the bottom part of scaffolding) 
3. jump in water and throw items under the scaffolding and watch them float up
4. items will start vibrating back and forth like they are stuck in a block when they reach the bottom part of the scaffolding and get flung in a random direction.

expected behavior:

Items should just float up through the scaffolding like it's just another water block.

Imgur Video of bug: https://i.imgur.com/kosnLVC.mp4

Tested and found in: 1.20.4 and 24w14a