The bug
You can't pick anything up if you stand in the water pipeline near a half block if you don't move (Your coordinates don't change). You can pick items up if you move, and you can't pick items up if you don't move.
How to reproduce
Create a water pipeline, it should contain any type of the half block at the end of the water flow. See the attachment.
Stand in the water pipeline, near the half block.
Use a dropper to put items in the pipeline.
If you move, you can pick items up. Otherwise you can't.
Plus: If any blocks placed at the top of the half block, you still can pick items up without moving yourself.
It seems to think the player's position is on top of the slab for some reason if they are capable of standing on it (i.e. no full blocks are in the two spaces above the slab), which would be why you don't pick up the item, and moving updates the position to the right spot thus allowing the item to be picked up.