Thanks @chapoi Updating helped.
Now nested threads have the correct number of posts - or at least the samples I have checked do.
The update process initially broke my site, apparently due to an old database import template reference in app.yml that was invoking mySQL12 - deleting that line and trying again resolved it.
- "templates/import/mysql-dep.template.yml"
Not sure why this happened, because Discourse has been successfully updated many times over many years with that line in place.
I wonder if something in a change added since the official full 2026.6.0-latest release (which update did not have any issues) has added some sort of new sensitivity?
Anyway, line no longer needed, and all good now.