Just remembered: I did not restart the npcap service after running the NPFInstall.exe commands, which was probably required in order for Npcap to know about the change. Full reinstall is probably not required, just the loopback uninstall and install, then restart the driver service.
I will be checking the upgrade logs for messages related to the Npcap Loopback Adapter to see why it was removed.
Legacy loopback support with the Npcap Loopback Adapter is now strongly deprecated. We do not intend to address the problem of Windows feature updates removing the adapter. Recommended fix is to reinstall without this feature.