The bug

When a block is a farmland block, the farmer jumps or falls on it, and the block of farmland is transfused into a block of dirt, then the farmer will plant seeds on it, even if it is not (anymore) a farmland block.

How to reproduce

1. Make a large field of crops
2. Place a farmer villager and a composter block in the middle of the field
3. Wait until the villager jumps on a seed (or jumps on the composter and then jumps on a seed), and the farmland block turns into a block of dirt.
4. Look at the behavior of the villager replanting the seed on the block of dirt

Note: This bug does not happen every time.

Examples

Here are examples in videos:
Farmers can plant seeds on a non-farmland dirt block (1).mp4
Farmers can plant seeds on a non-farmland dirt block (2).mp4