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.