I posted here a little while back, but I though I had solved my issue. But I hadn't.
To re-state it
post indexer is not writing pages to wp_site_posts
This is on a fresh & relatively uncomplex install in test today.
(I though I'd solved it because a saw an option in global site search, (all/posts) however of course that impact reading from the db not writing, I confused myself)
Can you assist?