1. VMware Cloud Foundation 5.1 Administration Guide, "vCenter Server in VMware Cloud Foundation" section.
This document states: "All vCenter Server instances in a VMware Cloud Foundation system are connected in Enhanced Linked Mode. You can log in to any vCenter Server instance and view and manage the inventories of all the vCenter Server instances in the group." This confirms that the behavior described in the question is abnormal and indicates a problem.
2. VMware vSphere 7.0 Documentation, vCenter Server and Host Management, "vCenter Enhanced Linked Mode" section.
This official documentation specifies: "To join vCenter Server systems in Enhanced Linked Mode, you must connect them to the same vCenter Single Sign-On domain." This directly connects the SSO configuration (Option A) to the functionality of a global inventory view. A failure in this process would lead to the described issue.
3. VMware Cloud Foundation 4.5 Architecture and Design Guide, "vCenter Server Architecture" section, page 18.
This guide details the SSO topology: "VMware Cloud Foundation implements a single vCenter Single Sign-On domain by default... When you create a VI workload domain, SDDC Manager joins the vCenter Server for the workload domain to the common vCenter Single Sign-On domain." This highlights that the SSO and linking configuration is the foundational step for cross-vCenter visibility.