A Zero-Tick Farm is a farm that relies on replacing the block a crop (which grows on top of itself) is resting on, which forces it to grow a stage. This bug however was patched in 20w12a and does not work in 1.16+.
Deleting the block that a plant is resting on will schedule a tile tick on some crops that don’t break instantly. It will then check 1 gt later if it should now break. However, in the case where the block has been replaced before this check, it will execute the rest of the tile tick that grows the plant, something that is normally only called when the plant block gets randomticked.