Really mind bending concept, I'd be interested in seeing how far you could take this.
It might be worth looking into how some things are communicated, like if the held boxes had a gravity indicator since it's pretty easy to get things mixed up and dropping a box in the wrong place can result in a softlock.
But overall this is very well made. Great work!
I didn't actually get softlocked, it's just something I realised could happen, but level 6 is a good candidate for likely softlocks. I noticed this fairly early and kept testing boxes in doorways so they wouldn't float off too far. It's not too much of an issue since you included a reset button though.
Also maybe the position the player is holding the box could change depending on the gravity? Like if the player and box have opposite gravity the box is positioned above the player's head, and otherwise it's around their torso.
