Infinite Scroll in generated themes doesn't load new posts

When generating a new theme, the default IS setup doesn't load new posts, either in scroll or scroll-to-click style. When you click on the button, no new posts are added to the page (in source or visibly) and the button disappears.

See it in (in)action:


The first theme is a "base theme" generated with the pattern library; the second theme is an _s theme generated by underscores.me.

I initially discovered this using the "business" type, but I tested with other types & it seems consistent across the different types.

Reproduce the issue in a generated base theme and an _s theme, using both scroll and scroll-to-click styles. Inspect the default infinite-scroll setup and verify that clicking or scrolling appends new posts to the page and leaves the control behaving correctly.

Written by the indexing model from the issue text.