The bug
The hitbox of crops on farmland is not properly aligned to the farmland. Instead, it looks as if the farmland had normal block height.
This is a screenshot of the bug happening:
[media]To reproduce
Stand on a slab in front of two dirt blocks on top of each other
Use a hoe on the upper dirt block to turn it into farmland
Place crops on it
Focus the crops and notice that the hitbox is floating
Confirmed for 1.13 & 18w30b.