According to the wiki, farmland without a water source is suppose to decay. This is normally the case in the nether, it will decay since water source blocks cannot exist in the nether hence neither can hydrated farmland.

However, there seems to be a bug that allows the player to quickly till the grass block and plant a seed, allowing the crop to grow. This should not be the case, the tilled block should immediately dry back to regular dirt upon any attempt to till it. Similar to how wet sponges instantly dry out or attempts to place water source blocks immediately evaporate in the nether.

This is likely an unintentional bug since tilled land not immediately planted upon will decay. It appears that a water source check is not being performed for this block in the nether.

Additionally, grass blocks should not be able to expand to adjacent dirt blocks in the nether for the same reason. No water in the nether = no ability for grass to expand to dirt blocks.

Expected behavior would be the following within the nether:

• Placed grass that is tilled will not hydrate hence will not allow planting of crops.

• Dirt placed beside grass blocks in the nether should not expand, and grass blocks having no means of staying hydrated should decay to dirt rapidly.

WIKI Reference for Farmland Hydration and Growing Conditions:

"A farmland block is created dry, and eventually reverts back to dirt if not hydrated and nothing is planted on it."

This would indicate that BOTH conditions must be true in order to allow planting. If either condition is false, it should not be plantable.