What I expected to happen was...:
To be able to plant an underground farm in the Mangrove Forest

What actually happened was...:
I tilled up all the dirt then started planting and these Tetris areas wouldn't take the seed and remained farmland blocks with no crops. 

Steps to Reproduce:
1. Dug out area beneath the ground in mangrove forest.

2. Replace floor with dirt and convert into farmland.
3. Plant seeds until a crop land doesn't take the seeds.