Thank you! That's very helpful, it made me understand better how defining the trigger works! I appreciate your support.
Thanks for pointing out that it wasn't the latest version of GSAP. I just added the one that came by default in Codepen - looks like they are linking to an older version.
EDIT: I just checked closer and unfortunately there is still an issue. As I scroll down, the boxes that are outside the group disappear as soon as the ones inside group start to appear.