Minecraft 1.13, Minecraft 18w43c, Minecraft 18w48b, Minecraft 19w04b, 1.15 Pre-release 1
How to recreate:
- Place 2 blocks of sand on top of each other.
- Pick a shovel and a stair/slab.
- Switch to gamemode survival.
- Put the slab/stair in your off-hand.
- Dig the lowest sand block and place the slab/stair as soon as the sand is broken.
What's expected to happen:
The sand falls onto the slab/stair, when a slab is used, the sand block breaks and drops itself as soon as it falls onto the slab.
The slab/stair is not placed and stays in the players inventory.
What actually happens:
If you place the stair/slab quickly enough, it disappears until you rejoin the world or place another item in your off-hand.