When I upgraded Jekyll from 2.5 to 3, the pages that iterate over site.posts had no content in them. A deprecation warning points the way towards the fix.

$ bundle exec jekyll build
Configuration file: /pipeline/source/_config.yml
       Deprecation: You appear to have pagination turned 
       on, but you haven't included the `jekyll-paginate`
       gem. Ensure you have `gems: [jekyll-paginate]` in
       your configuration file.
            Source: /pipeline/source

I added gems: [jekyll-paginate] to my config file but when I tried to build another error surfaced:

`...require': cannot load such file -- jekyll-paginate (LoadError)

I had misread the original as jekyll-pagination which is already in the Gemfile. I added jekyll-paginate, then ran bundle update and all was well again.