Okay, I managed to find the issue and it'll be solved in the next version. Thanks for pointing it out!
What's going on is that there's another box right in front of Mao that only shows up in chapter 7 (which released in v0.7.0). The box event is still there but invisible and impossible for the player to interact. The issue is that NPCs, unlike the player, can't walk through any events, even ones that haven't appeared yet. So in the cutscene Cindy was stuck trying to walk through a box that only appears in chapter 7 (which is why going back to v0.6.1, when the box didn't yet exist, solved it for you). The only way to find this kind of issue would be if I replayed all the previous chapters whenever a new one came out, which I didn't. So sorry for the trouble ;-;
Long story short, it's just the RPG Maker engine being dumb and me not being careful enough to imagine an asset added in chapter 7 would conflict with a cutscene in chapter 5. So thanks again for pointing that out!