TImebox to 1 day investigation. When no result then simply document the issue.

In aa SPA project: This is written to the log whenever opening the "Preview as Visitor" app.

WARN  ation.preview.ui.component.IFrameNavigationTracker: iframe_navigation_tracker_connector.js script couldn't resolve iframe location. Possibly missing <code>cms:page</code> tag or used deprecated <code>cms:init</code> tag in the template.

The warning should not be written to the log I think. As everything seems to work.

This warning will confuse developers as they work on a project as they will wonder what is wrong in their project or templates, they will search for a solution but not find one.