dataSlayer panel not displaying dataLayer - only GA hits (acts as if there's no GTM on page)

[CyBrian]

On the site I'm working on, the problem only occurs on the page with an iframe.
See attached screenshot

When opening the iframe directly, as the main page (only a top window - no iframe), dataSlayer displays normally, including GTM icon.

Both top and iframe load container "GTM-5*"
The iframe also loads a 2nd container "GTM-W*"
(The 2nd is a subset of the first and will be removed following some data QA)

dataLayer in top window

0:{gtm.start: 1525112388999, event: "gtm.js", gtm.uniqueEventId: 0}
1:{event: "CDsAssigned", GTMidVersion: "GTM-5* | 39", userAgent: "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWeb…L, like Gecko) Chrome/66.0.3359.139 Safari/537.36", gtm.uniqueEventId: 4}
2:{event: "gtm.dom", gtm.uniqueEventId: 8}
3:{event: "ScrollDistance", eventCategory: "Scroll Depth", eventAction: "Percentage", eventLabel: "Baseline", eventValue: 1, …}
4:{event: "gtm.load", gtm.uniqueEventId: 10}
etc....

dataLayer in iframe

0:{gtm.start: 1525112389934, event: "gtm.js", gtm.uniqueEventId: 0}
1:{gtm.start: 1525112389935, event: "gtm.js", gtm.uniqueEventId: 1}
2:{event: "gtm.dom", gtm.uniqueEventId: 2}
3:{event: "ScrollDistance", eventCategory: "Scroll Depth", eventAction: "Percentage", eventLabel: "Baseline", eventValue: 1, …}
4:{event: "CDsAssigned", GTMidVersion: "GTM-5* | 39", userAgent: "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWeb…L, like Gecko) Chrome/66.0.3359.139 Safari/537.36", gtm.uniqueEventId: 7}
5:{event: "ScrollDistance", eventCategory: "Scroll Depth", eventAction: "Percentage", eventLabel: "Baseline", eventValue: 1, …}
6:{event: "gtm.load", gtm.uniqueEventId: 11}
etc....

[CyBrian]

Hi Sean
I read through other open issues to anticipate questions and confirm seeing no errors in dataSlayer's console
Thanks
Brian