It turns out that place/taxon notifications are configured to send notifications of observations descended from the selected taxon but not at the selected taxon. It has likely been this way for several years, without detection.
I am trying to determine how the planned changes to subscriptions and notifications will impact this kind of notification to determine whether to fix as-is or in the context of the other changes.
In the meantime, if some of the subscriptions would be appropriate to change to genus (I know many aren’t), that would trigger notifications.