Google Tag Manager / Enhanced E-commerce - Pageview tag dataLayer push not working

On one website I have completed a successful Enhanced E-commerce integration with GTM using dataLayer and all tags are firing, data is coming through to Google Analytics as expected. However, on another cloned website, I have also setup with GTM/GA in exactly the same way, however the dataLayer push is not sending product detail views or product listing impressions.

I enabled Enhanced E-commerce features in the GA view, using a basic page view tag in GTM, which again is configured in exactly the same way as the working site, like so:

and then pushing into the dataLayer object at the bottom of the script:

    dataLayer.push({
        'ecommerce': {
            'currencyCode': 'GBP',
            'impressions': gtmProducts
        }
    });

In Google Analytics Debugger v2.7 I see the following output - no product impressions transmitted on the page view!

                         _                          _       _   _
                        | |                        | |     | | (_)
  __ _  ___   ___   __ _| | ___    __ _ _ __   __ _| |_   _| |_ _  ___ ___
 / _` |/ _ \ / _ \ / _` | |/ _ \  / _` | '_ \ / _` | | | | | __| |/ __/ __|
| (_| | (_) | (_) | (_| | |  __/ | (_| | | | | (_| | | |_| | |_| | (__\__ \
 \__, |\___/ \___/ \__, |_|\___|  \__,_|_| |_|\__,_|_|\__, |\__|_|\___|___/
  __/ |             __/ |                              __/ |
 |___/             |___/                              |___/

analytics_debug.js:15 Running analytics_debug.js. This script is intended for testing and debugging only.
log @ analytics_debug.js:15
J @ analytics_debug.js:14
(anonymous) @ analytics_debug.js:101
(anonymous) @ analytics_debug.js:101
analytics_debug.js:15 Initializing Google Analytics.
analytics_debug.js:15 Loading resource for plugin: ec
analytics_debug.js:15 Loading script: "https://www.google-analytics.com/plugins/ua/ec.js"
analytics_debug.js:15 Running command: ga("create", "my-trackingId", {name: "gtm1", cookieDomain: "auto"})
analytics_debug.js:15 Creating new tracker: gtm1
analytics_debug.js:15 Auto cookieDomain found: "my-domain"
analytics_debug.js:15 Running command: ga("gtm1.set", "&gtm", "my-gtm")
analytics_debug.js:15 Running command: ga("gtm1.set", "hitCallback", [function])
analytics_debug.js:15 Running command: ga("gtm1.require", "ec", "ec.js")
analytics_debug.js:15 Waiting on require of "ec" to be fulfilled.
analytics_debug.js:15 Registered new plugin: ga(provide, "render", Function)
analytics_debug.js:15 Running command: ga("gtm1.require", "ec", "ec.js")
analytics_debug.js:15 Waiting on require of "ec" to be fulfilled.
analytics_debug.js:15 Executing Google Analytics commands.
analytics_debug.js:15 Registered new plugin: ga(provide, "ec", Function)
analytics_debug.js:15 Running command: ga("gtm1.require", "ec", "ec.js")
analytics_debug.js:15 Plugin "ec" intialized on tracker "gtm1".
analytics_debug.js:15 Running command: ga("gtm1.send", "pageview")
analytics_debug.js:15 
Sent beacon:
v=1&_v=j76d&a=my-adSenseId&t=pageview&_s=1&dl=http%3A%2F%2Fmy-domain%2Fshopby%2Frings&ul=en-us&de=UTF-8&dt=Title%20%7C%20My%20Site&sd=24-bit&sr=2560x1440&vp=2545x714&je=0&_u=SCCAAEAL~&jid=&gjid=&cid=my-clientId&tid=my-trackingId&_gid=1488000190.1561364013&gtm=my-gtm&z=134019913


analytics_debug.js:15 <unknown>        (&gtm)  my-gtm
analytics_debug.js:15 _j1              (&jid)  
analytics_debug.js:15 _j2              (&gjid) 
analytics_debug.js:15 adSenseId        (&a)    my-adSenseId
analytics_debug.js:15 apiVersion       (&v)    1
analytics_debug.js:15 clientId         (&cid)  my-clientId
analytics_debug.js:15 encoding         (&de)   UTF-8
analytics_debug.js:15 hitType          (&t)    pageview
analytics_debug.js:15 javaEnabled      (&je)   0
analytics_debug.js:15 language         (&ul)   en-us
analytics_debug.js:15 location         (&dl)   http://my-domain/shopby/rings
analytics_debug.js:15 screenColors     (&sd)   24-bit
analytics_debug.js:15 screenResolution (&sr)   2560x1440
analytics_debug.js:15 title            (&dt)   Title
analytics_debug.js:15 trackingId       (&tid)  my-trackingId
analytics_debug.js:15 viewportSize     (&vp)   2545x714

However, if I modify this push code to be triggered on a custom EEproductListingsUpdate event Tag like so:

    dataLayer.push({
        'event': 'EEproductListingsUpdate',
        'ecommerce': {
            'currencyCode': 'GBP',
            'impressions': gtmProducts
        }
    });

With the following Tag configuration:

Then I see the following command in GA Debugger:

Running command: ga("gtm3.send", {hitType: "event", eventCategory: "eCommerce", eventAction: "Product Listings Update", eventLabel: undefined, eventValue: undefined})

And the impressions are all sent... so why might it be that the Pageview Tag does not work to transmit the impressions when the dataLayer.push() occurs like on the other site? Where should I be looking to find the answer when code-wise and config-wise in the GA/GTM panels, EVERYTHING is the same?