SpriteNodes are disappearing randomly upon collision

Question

I'm creating an iOS iPhone game similar to tetris where blocks fall from the sky and stack up on each other. However, my SpriteNodes blocks randomly disappear sometimes when a block comes on top of another one.

This is my block placement, I call this for 10 iterations to place random blocks that fall:

-(void)setupBlocksForT:(float)time {

    Block *block = (Block *)[SKSpriteNode spriteNodeWithImageNamed:@"block1"];
    block.physicsBody = [SKPhysicsBody bodyWithTexture:block.texture size:block.size];
    block.physicsBody.dynamic = YES;
    block.physicsBody.allowsRotation = NO;
    block.physicsBody.usesPreciseCollisionDetection = YES;
    block.physicsBody.restitution = 0;
    block.physicsBody.friction = 1.0;
    block.physicsBody.categoryBitMask = BlockMask;
    block.physicsBody.collisionBitMask = CharacterMask | GroundMask | BlockMask;
    block.physicsBody.contactTestBitMask = BlockMask | CharacterMask;

    double randomSize = rand_range_double(0.10, 0.5);
    block.xScale = randomSize;
    block.yScale = randomSize;

    block.position = CGPointMake(rand_range_int(0, self.frame.size.width, self.frame.size.height);

    SKAction *wait = [SKAction waitForDuration:1.5 * time];
    SKAction *spawn = [SKAction runBlock:^{
        [self.myWorld addChild: block];

    }];

    SKAction *spawnSequence = [SKAction sequence:@[wait, spawn]];

    [self runAction:spawnSequence];
}

Nowhere in my code do I call a remove(node).

I'm developing using SpriteKit and my setup is having an SKNode that contains all of my blocks so that the camera stays focused on my character's position in the DidSimulatePhysics() method.

Has anyone run into an issue where their sprites are randomly disappearing?