I'm facing a similar issue. My iPhone 14 is running iOS 18.4, while my iPhone 16E is stuck on iOS 18.3.1 and won’t update to the developer beta. As a result, I can't transfer data to the new iPhone since Apple prevents migrations from a device with a newer iOS to one with an older version—whether through iOS, macOS Finder, or iMazing. The irony is that the newer device is running an older OS.