The bug

When an upwards facing dispenser uses bone meal on a grass block it requires an air block above the targeted block for plants like tall grass and flowers to generate.

In 1.13 a solid or transparent block above the targeted grass block allowed plants to grow.

In 1.13.1 and above an air block is required above the grass block directly above the dispenser.

Steps to reproduce

1. Open any Minecraft version newer than 1.13
2. Place an upwards facing dispenser and add bone meal
3. Place grass blocks at the Y-level directly above the dispenser
4. Place a non-air block two blocks above the dispenser, one block above the grass.
5. Trigger the dispenser