The generalized iteration feature is undergoing limited smoke testing on production which is why you happened to see it work in a live game, but it isn’t released yet, so there’s no guarantee that it will keep working or work on every platform.
Please wait for an announcement on it before using it.
The reason that you unfortunately don’t see an error in the Studio script editor is that for ... in SOMETHING do is always at least syntactically valid for any SOMETHING even though it may or may not actually do what you intend depending what the SOMETHING you substitute in is (and whether the new iteration semantics are released).