I have a setup where a Drupal site sets a data layer in the HTML that, as far as I have tested and debugged, correctly sets keys and values. I then have the site connected to GA4 through a Tag Manager tag, and the data is then sent to BigQuery for storage. For months, this setup was seemingly fine and we were happy with the accuracy of the data we were collecting and querying in BigQuery.
However, recently we're starting to see blank cells where custom data should be. Google is clearly detecting page views as the page view row is recorded and also picks up it's own definitions like page title, device etc.
No matter what I test and preview in Tag Manager (in terms of different users, user types, content type etc), I cannot replicate a situation where the data isn't being set and detected correctly.
What possible reasons could prevent custom data in a datalayer from reaching GA4 via Tag Manager, when Google's own definitions are detected without issue?