In short, the AI is incorrect. Let's pretend that ChatGPT doesn't exist for a second and do some old-fashioned research, starting with the page for...in on MDN. It reads:

The traversal order, as of modern ECMAScript specification, is well-defined and consistent across implementations. Within each component of the prototype chain, all non-negative integer keys (those that can be array indices) will be traversed first in ascending order by value, then other string keys in ascending chronological order of property creation.

So, the concern that the order of the keys is inconsistent between JS environments seems to be invalid, at least in the current day. Other reliable resources such as this page seems to suggest that keys have been well-ordered as part of the specification since ES2015; and caniuse reports that 96.57% of browsers implement that version of the spec.

I've always done it this way, and it has always worked. All the test cases I tried passed.

That's not surprising. You are almost certainly using an ES2015 compliant environment and so the traversal order is the same as the insertion order. In this case, the keys are inserted in the order that they appear in the string.

Was the AI possibly mistaken?

Yes.