Please read comments from customer:

after bisecting frontend-helpers master branch I found the actual source of our issue - which underlines my initial guess, that magnolia is not notified correctly about change on the frontend.

I found, that our problem arises from frontend-helpers#5528322c51017edc45fab032ebfd1db93bab8830 (before react-editor works fine).

My guess is, that a regression was introduced by removing the line packages/react-editor/src/component/EditablePage/EditablePage.js?f=52 with statement EditorContextHelper.onFrameReady().

Somehow, this method is supposed to inform magnolia about some form of change when the page is running in an iframe, which then is triggering Magnolia to update the preview button with a location url.

After re-adding the code line to the mentioned commit, our problem does not occur anymore. I got the same result, when re-adding the code to the latest commit in branch master.

Maybe you can bring some light into the matter, if this is the correct reasoning, and why this does not happen when running an external SPA?