craftbukkit/nms-patches/net/minecraft/world/level/block/BlockWitherSkull.patch

--- a/net/minecraft/world/level/block/BlockWitherSkull.java
+++ b/net/minecraft/world/level/block/BlockWitherSkull.java
@@ -26,6 +26,10 @@
 import net.minecraft.world.level.block.state.pattern.ShapeDetectorBuilder;
 import net.minecraft.world.level.block.state.predicate.BlockStatePredicate;
 
+// CraftBukkit start
+import org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason;
+// CraftBukkit end
+
 public class BlockWitherSkull extends BlockSkull {
 
     public static final MapCodec<BlockWitherSkull> CODEC = simpleCodec(BlockWitherSkull::new);
@@ -58,6 +62,7 @@
     }
 
     public static void checkSpawn(World world, BlockPosition blockposition, TileEntitySkull tileentityskull) {
+        if (world.captureBlockStates) return; // CraftBukkit
         if (!world.isClientSide) {
             IBlockData iblockdata = tileentityskull.getBlockState();
             boolean flag = iblockdata.is(Blocks.WITHER_SKELETON_SKULL) || iblockdata.is(Blocks.WITHER_SKELETON_WALL_SKULL);
@@ -69,12 +74,18 @@
                     EntityWither entitywither = (EntityWither) EntityTypes.WITHER.create(world, EntitySpawnReason.TRIGGERED);
 
                     if (entitywither != null) {
-                        BlockPumpkinCarved.clearPatternBlocks(world, shapedetector_shapedetectorcollection);
+                        // BlockPumpkinCarved.clearPatternBlocks(world, shapedetector_shapedetectorcollection); // CraftBukkit - move down
                         BlockPosition blockposition1 = shapedetector_shapedetectorcollection.getBlock(1, 2, 0).getPos();
 
                         entitywither.moveTo((double) blockposition1.getX() + 0.5D, (double) blockposition1.getY() + 0.55D, (double) blockposition1.getZ() + 0.5D, shapedetector_shapedetectorcollection.getForwards().getAxis() == EnumDirection.EnumAxis.X ? 0.0F : 90.0F, 0.0F);
                         entitywither.yBodyRot = shapedetector_shapedetectorcollection.getForwards().getAxis() == EnumDirection.EnumAxis.X ? 0.0F : 90.0F;
                         entitywither.makeInvulnerable();
+                        // CraftBukkit start
+                        if (!world.addFreshEntity(entitywither, SpawnReason.BUILD_WITHER)) {
+                            return;
+                        }
+                        BlockPumpkinCarved.clearPatternBlocks(world, shapedetector_shapedetectorcollection); // CraftBukkit - from above
+                        // CraftBukkit end
                         Iterator iterator = world.getEntitiesOfClass(EntityPlayer.class, entitywither.getBoundingBox().inflate(50.0D)).iterator();
 
                         while (iterator.hasNext()) {
@@ -83,7 +94,7 @@
                             CriterionTriggers.SUMMONED_ENTITY.trigger(entityplayer, (Entity) entitywither);
                         }
 
-                        world.addFreshEntity(entitywither);
+                        // world.addFreshEntity(entitywither); // CraftBukkit - moved up
                         BlockPumpkinCarved.updatePatternBlocks(world, shapedetector_shapedetectorcollection);
                     }