Usually on observable there are some types, that for some reason error when calling toString on them, when this happens an error is thrown on the promise and the observable cell is never updated to a new value again.
So if you want to save this special variable on a cell, this is imposible, even if you wrap it in an array or on a object.
This happens in firefox. If you want an example of a notebook where this issue happens visit this one.
Edit: it seems you guys have fixed it.