Initial beta versions of iOS 16 and iPadOS 16 had noticeable change in Home app. iPad was not eligible to act as Home Hub anymore.
Apple made a comment that iPad could be used as Home hub for "current features" while undisclosed "new features", probably won't be supported.
As of now, it seems that these "new features" are related to Matter standard which HomeKit is adopting.
Reference: iPads Will Work as Home Hubs in iOS 16, But Won't Support New Home Architecture