Farmland near water does not turn to dirt when a block is placed on top of it

Steps to Reproduce:

1. Dig a one block deep hole
2. Place water in the hole
3. Till a grass block next to the water
4. Place a block on top of the farmland
5. Break the block on top of the farmland

Observed Results:
The farmland does not turn to dirt.

Expected Results:
The farmland should turn to dirt.

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Original Description:
Place a block on a tilled dirt block it should turn back to normal dirt but I did it in my survival world it stayed tilled please help