Jekyll paginator = nil on post.html but not index.html

I'm using Jekyll 4.4.1 and jekyll-paginate-v2. My home page template (index.html) has an active paginator object, but my individual post page template (_layouts/post.html) does not (paginator = nil).

My _config.yml has

pagination:  enabled: true  per_page: 4  extension:  indexpage:  sort_reverse: true  trail:    before: 2    after: 2

My index.html layout has

pagination:  enabled: true

And my posts.html layout has

pagination:  enabled: true

What could I have set up wrong that pagination works fine with index.html but not posts.html?