Anvils aren't non-solid, they are transparent

solid and transparent are not mutually exclusive!

src/pocketmine/block/Anvil.php

@@ -38,14 +38,14 @@ class Anvil extends Fallable{
 
 	protected $id = self::ANVIL;
 
-	public function isSolid() : bool{
-		return false;
-	}
-
 	public function __construct(int $meta = 0){
 		$this->meta = $meta;
 	}
 
+	public function isTransparent() : bool{
+		return true;
+	}
+
 	public function getHardness() : float{
 		return 5;
 	}