I made a farm underground at level 56 and tried to place potatoes. There was enough light, farmland and a watersource. I was able to plant on most of the farmland, but I couldn't place them on certain blocks. The block were irregular, so not in a specific pattern. There were 4 blocks of air abobe every farmland. I was confused and tried it on another place on the same level with the same condiions. I tested it if I can plant there by getting 9 potatoes and try to plant it on every row individually. It worked on every space. Then when I placed them on every block, I couldn't plant on three farmland. Very frustrating. Two sides of the farm had walls to the ceiling. When I mined one of the walls, four crops broke i an irregular shape.

This happened on a private server with potential mods and Optifine, so I tried to recreae it in a singleplayer vanilla world and it happened again.