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Can only give you a theoretical answer - don't have that much experience with debugging the tag manager. From the "manual":

Data layer needs to be above the container snippet
Datalayer should be added above the container snippet. Variables pushed to the data layer after the container snippet will not be able to fire tags on page loads with a matching condition.
Variables pushed to the data layer after the container snippet will not be able to fire tags on page loads with a matching condition.
Move the datalayer to above the container snippet

Check also https://www.optimizesmart.com/google-tag-manager-data-layer-explained-like-never/ - it seems that you define your data layer after the position of the tag - so the tag cannot send the info from the data layer to Analytics. Check your source code & check where the data layer is defined. Probably it's ok on your category pages but not on the detail pages (compare both types to see the difference)

Hope this helps.

Dirk

Apart from the Tag manager assistant - you could also check the debugger -https://chrome.google.com/webstore/detail/google-analytics-debugger/jnkmfdileelhofjcijamephohjechhna?hl=en-US  - like Martijn mentioned there should be an error somewhere - did you check the messages in the console?

Are you sure that there are no JavaScript errors on the page?

Hmm yeah that seems right to me!

What Dirk is suggesting would be the first thing to do in my opinion as well. But the second one is what trigger are you using in Google Tag Manager. Are you targeting all pages in there or something else?

Run Screaming Frog - and check if the tag id is present on all pages. Apart from checking - you could also extract the code from the source to double check that the ID is correct; check here how you can do this

In Analytics reporting (Behavior > Site Content > Content Drill down) set the day after migration & compare it to the day before migration - check if there if the drop is generic (=all pages) or linked to a few sections. If the drop is higher in specific sections - go down to next level & do the same. Check the pages with the highest traffic loss using the Tag Assistant plugin for Chrome. It checks if the tag is properly implemented on these pages (https://chrome.google.com/webstore/detail/tag-assistant-by-google/kejbdjndbnbjgmefkgdddjlbokphdefk)

You can also use the Tag Assistant plugin on your main pages to see if it is sending data as you would expect.

Hope this helps.

Dirk