How to recreate:

1. Place 2 blocks of sand on top of each other.
2. Pick a shovel and a stair/slab.
3. Switch to gamemode survival.
4. Put the slab/stair in your off-hand.
5. 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.
OR
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.