the bug:

when an item entity lands on the edge of block where water is floating on, it can't get pushed bu the water current like what you'd expect.

how to reproduce:

1. create a test world and build the test contraption in image 1. the player should be facing north, or toward negative z

2. paste those commands in the command block

/summon minecraft:item ~ ~-1 ~0.4 {Item:{id:"minecraft:glass",Count:1b}}

/summon minecraft:item ~ ~-1 ~0.623 {Item:{id:"minecraft:glass",Count:1b}}

/summon minecraft:item ~ ~-1 ~0.624 {Item:{id:"minecraft:glass",Count:1b}}

/summon minecraft:item ~ ~-1 ~0.625 {Item:{id:"minecraft:glass",Count:1b}}

3. stand a few blocks away from the contraption, and activate the command blocks.

4. the first and second item gets pushed down by the water stream, the third rests on the very edge of the stair block, not pushed by the water and falling down, the last just drops down directly. (video in the attachment file)