Recap:
I created pages from an external API using a custom plugin.
The plugin itself uses a custom generator to transform and build pages based on data.
On the data pages, I could access page data {page.custom_property}
Outside of the data pages, I could not access this data from site.pages
This is because the standard site.pages is already set by the time this generator runs.
Solution:
In my plugin, I simply added my page data to site.data instead, since it is accessible at anytime.