WOW! That game is awesome!
You have found a bug! I’ve opened an issue for it here: https://github.com/microsoft/pxt-arcade/issues/1737
Basically, the issue is that the open chest tile doesn’t appear anywhere in the tilemap.
You can work around it by opening your tilemap, adding an open chest, and closing the tilemap editor. You can probably even delete the open chest from the map after is has been added at least once.