What's the context ?
"works <-> doesn't"
What, if any, are the error messages ?
What kind of value are you trying to obtain ?
I think you would have to post a full KM example to make it clear what is really being evaluated, and in what interpreter and browser context.
In the meanwhile, you will get more helpful error messages, and a temporary local namespace, rather than a global one which persists between script runs, if you enclose your code in a "use strict" IIFE context.
If you have bound the name iFrame with let or const in a global namespace in one script run, then it will remain defined (unavailable for redefinition) in the next script run.
Polluting the global namespace is never a good plan.
(() => {
"use strict";
// Code to evaluate here, in the temporary namespace
// roped off by an Immediately Invoked Function Expression
// ...
})()
See: