I know reusable blocks are WP core, but I’ve uninstalled / deactivated all plugins and turned them on 1 by 1. It’s GP Premium (even when it’s the only activated) that’s preventing reusable components.
UPDATE: I deleted all of my reusable blocks (accessed from the “manage reusable blocks” hover menu when selecting a reusable block in the page editor). Then it appeared.
Must have been something in one of them that it didn’t like!