GTM preview does not work in an embedded app

[Zet]

Hello,

We've managed to integrate GTM in our embedded application but we're having trouble using the Preview functionality. We want to be able to use it to test out our tags without having to wait for the changes to show up live which causes headaches.

The problem is that the preview does not pick up the GTM tag in the iframe. We know it works, because it's sending the already deployed events, but the iframe itself prevents the GTM tag from being picked up by the Tag Assistant Companion. And yes, we are using the extension, as it's said in the Google docs:

The page loads the Google tag in an iframe. iframe-loaded tag configurations can only be debugged when the Tag Assistant Companion chrome extension is also installed.

But it does not work with it either. Has anyone been able to make the preview work for embedded applications development?