This code will: Recreate lost signs, furnaces and chests, and remove invalid or corrupt tile entities from a world.

worldFixer.php

<?php
$world = $this->getServer()->getDefaultLevel();
$radius = 80;
$total = (($radius * 2) + 1) ** 2;
$count = 0;
for($chunkX = -$radius; $chunkX <= $radius; ++$chunkX){
	for($chunkZ = -$radius; $chunkZ <= $radius; ++$chunkZ){
		$chunk = $world->getChunk($chunkX, $chunkZ, false);
		if($chunk instanceof \pocketmine\level\format\FullChunk and $chunk->isPopulated()){
			$tiles = [];
			$save = false;
			foreach($chunk->getTiles() as $tile){
				$x = $tile->x & 0x0f;
				$y = $tile->y;
				$z = $tile->z & 0x0f;
				$blockId = $chunk->getBlockId($x, $y, $z);
				if(isset($tiles["$x:$y:$z"])){
					echo "Found duplicated tile in chunk $chunkX, $chunkZ [$x:$y:$z]\n";
					$tile->close();
					$save = true;
				}elseif($tile instanceof \pocketmine\tile\Chest){
					if($blockId !== 54){
						echo "Found corrupt chest in chunk $chunkX, $chunkZ [$x:$y:$z]\n";
						$tile->close();
						$save = true;
					}
				}elseif($tile instanceof \pocketmine\tile\Sign){
					if($blockId !== 63 and $blockId !== 68){
						echo "Found corrupt sign in chunk $chunkX, $chunkZ [$x:$y:$z]\n";
						$tile->close();
						$save = true;
					}
				}elseif($tile instanceof \pocketmine\tile\Furnace){
					if($blockId !== 61 and $blockId !== 62){
						echo "Found corrupt furnace in chunk $chunkX, $chunkZ [$x:$y:$z]\n";
						$tile->close();
						$save = true;
					}
				}
				$tiles["$x:$y:$z"] = true;
			}
			for($x = 0; $x < 16; ++$x){
				for($z = 0; $z < 16; ++$z){
					for($y = 0; $y < 128; ++$y){
						if(!isset($tiles["$x:$y:$z"])){
							$blockId = $chunk->getBlockId($x, $y, $z);
							if($blockId === 54){
								echo "Found non-tile chest in chunk $chunkX, $chunkZ [$x:$y:$z]\n";
								$nbt = new \pocketmine\nbt\tag\Compound(false, [
									new \pocketmine\nbt\tag\Enum("Items", []),
									new \pocketmine\nbt\tag\String("id", \pocketmine\tile\Tile::CHEST),
									new \pocketmine\nbt\tag\Int("x", $chunkX * 16 + $x),
									new \pocketmine\nbt\tag\Int("y", $y),
									new \pocketmine\nbt\tag\Int("z", $chunkZ * 16 + $z)
								]);
								$nbt->Items->setTagType(\pocketmine\nbt\NBT::TAG_Compound);
								new \pocketmine\tile\Chest($chunk, $nbt);
								$save = true;
							}elseif($blockId === 61 or $blockId === 62){
								echo "Found non-tile furnace in chunk $chunkX, $chunkZ [$x:$y:$z]\n";
								$nbt = new \pocketmine\nbt\tag\Compound(false, [
									new \pocketmine\nbt\tag\Enum("Items", []),
									new \pocketmine\nbt\tag\String("id", \pocketmine\tile\Tile::FURNACE),
									new \pocketmine\nbt\tag\Int("x", $chunkX * 16 + $x),
									new \pocketmine\nbt\tag\Int("y", $y),
									new \pocketmine\nbt\tag\Int("z", $chunkZ * 16 + $z)
								]);
								$nbt->Items->setTagType(\pocketmine\nbt\NBT::TAG_Compound);
								new \pocketmine\tile\Furnace($chunk, $nbt);
								$save = true;
							}elseif($blockId === 63 or $blockId === 68){
								echo "Found non-tile sign in chunk $chunkX, $chunkZ [$x:$y:$z]\n";
								$nbt = new \pocketmine\nbt\tag\Compound(false, [
									new \pocketmine\nbt\tag\String("id", \pocketmine\tile\Tile::SIGN),
									new \pocketmine\nbt\tag\Int("x", $chunkX * 16 + $x),
									new \pocketmine\nbt\tag\Int("y", $y),
									new \pocketmine\nbt\tag\Int("z", $chunkZ * 16 + $z)
								]);
								new \pocketmine\tile\Sign($chunk, $nbt);
								$save = true;
							}
						}
					}
				}
			}
			if($save){
				$world->setChunk($chunkX, $chunkZ, $chunk, false);
				$world->saveChunks();
			}
		}
		$world->unloadChunk($chunkX, $chunkZ, false);
		++$count;
	}
	echo "[".round(($count/$total) * 100, 0)."%] $count/$total\n";
}
$world->save(true);