Farmland or dirt path cannot be created below a light block

Can confirm. Also affects 21w14a.

Can confirm in 21w15a.

Can confirm in 21w16a.

Can confirm in 21w17a.

because lighjt blocks are full blocks with hitboxes and you can't till farmland under a block

I can confirm this behavior in 1.18.

Here's a code analysis of this issue. The following is based on a decompiled version of Minecraft 1.18 using MCP-Reborn. Please note that I'm quite tentative about this analysis, although I'm persuaded to believe that this is likely the cause of the problem. 🙂

Code Analysis (tentative):

net.minecraft.world.item.HoeItem.java

public class HoeItem extends DiggerItem {
   protected static final Map<Block, Pair<Predicate<UseOnContext>, Consumer<UseOnContext>>> TILLABLES = Maps.newHashMap(ImmutableMap.of(Blocks.GRASS_BLOCK, Pair.of(HoeItem::onlyIfAirAbove, changeIntoState(Blocks.FARMLAND.defaultBlockState())), Blocks.DIRT_PATH, Pair.of(HoeItem::onlyIfAirAbove, changeIntoState(Blocks.FARMLAND.defaultBlockState())), Blocks.DIRT, Pair.of(HoeItem::onlyIfAirAbove, changeIntoState(Blocks.FARMLAND.defaultBlockState())), Blocks.COARSE_DIRT, Pair.of(HoeItem::onlyIfAirAbove, changeIntoState(Blocks.DIRT.defaultBlockState())), Blocks.ROOTED_DIRT, Pair.of(($$0) -> {
      return true;
   }, changeIntoStateAndDropItem(Blocks.DIRT.defaultBlockState(), Items.HANGING_ROOTS))));
   ...
   public static boolean onlyIfAirAbove(UseOnContext $$1) {
      return $$1.getClickedFace() != Direction.DOWN && $$1.getLevel().getBlockState($$1.getClickedPos().above()).isAir();
   }
}

net.minecraft.world.item.ShovelItem.java

public class ShovelItem extends DiggerItem {
   protected static final Map<Block, BlockState> FLATTENABLES = Maps.newHashMap((new Builder()).put(Blocks.GRASS_BLOCK, Blocks.DIRT_PATH.defaultBlockState()).put(Blocks.DIRT, Blocks.DIRT_PATH.defaultBlockState()).put(Blocks.PODZOL, Blocks.DIRT_PATH.defaultBlockState()).put(Blocks.COARSE_DIRT, Blocks.DIRT_PATH.defaultBlockState()).put(Blocks.MYCELIUM, Blocks.DIRT_PATH.defaultBlockState()).put(Blocks.ROOTED_DIRT, Blocks.DIRT_PATH.defaultBlockState()).build());
   ...
   public InteractionResult useOn(UseOnContext $$0) {
      Level level = $$0.getLevel();
      BlockPos blockpos = $$0.getClickedPos();
      BlockState blockstate = level.getBlockState(blockpos);
      if ($$0.getClickedFace() == Direction.DOWN) {
         return InteractionResult.PASS;
      } else {
         Player player = $$0.getPlayer();
         BlockState blockstate1 = FLATTENABLES.get(blockstate.getBlock());
         BlockState blockstate2 = null;
         if (blockstate1 != null && level.getBlockState(blockpos.above()).isAir()) {
            level.playSound(player, blockpos, SoundEvents.SHOVEL_FLATTEN, SoundSource.BLOCKS, 1.0F, 1.0F);
            ...

If we look at the above classes, we can see that you can only convert blocks using shovels and hoes if air is above them. This is evident through the following line of codes:

net.minecraft.world.item.HoeItem.java

HoeItem::onlyIfAirAbove

net.minecraft.world.item.ShovelItem.java

if (blockstate1 != null && level.getBlockState(blockpos.above()).isAir())

Can confirm in 1.18.1.

Can confirm in 1.18.2.

Can confirm in 1.19.

Can confirm in 1.19.2.